The official Mike Bellm's

Bellm TCs


TC Contender, G2, Encore/ProHunter Performance Center

 




The chamber throat is THE single most important aspect of the chamber, yet it is the most neglected aspect of the chamber.

Chamber casts show what to expect by way of throat design when you buy a barrel chambered for a particular round.,

Some designs are good. Many are not. The shape of the throat can be a very good indication of what to expect by way of accuracy.

There are 3 basic throat configurations found in barrels

1) BEST.
Cylindrical throats of close diameter tolerance support the shank of the bullet as it is engraved by the rifling. Most bottle neck chambers are of this type, but the factory SAAMI spec. throat diameters are nearly always much larger than ideal and very often misaligned with the bore.

2) VERY POOR, MAY RIVAL THE 3RD TYPE FOR WORST.
Cone shaped throats do nothing to positively align the shank of the bullet with the bore. Virtually all straight wall handgun type chambers, more specifically .357 Magnum, .357 Maximum, .44 Magnum, .454 Casull, .460 S&W.

3) WORST.
No throat leaves only the case and chamber neck with .003" to about .010" clearance between them and the abrupt ends of the rifling to herd the bullet down the bore. Accuracy from this type of chamber is most often dismal to mediocre at best. Common examples are factory .45/70 chambers and some vintages of .30/30 chambers.

Learn what type of throat to expect and what to do about it to correct it.

This is what this page is largely about.


Short Course in "Chamber Throats 101"

We first need to define exactly what a true chamber throat is and what is supposed to do.

A "throat" in a chamber is a place where the bullet can extend out of the case neck and up into the barrel.

A bullet cannot extend up into the rifled area of the barrel due to the riflings being in the way. Riflings stick up from .002" .17 and .20 cal., 0025" for .22 cal., .0035" for 6mm and .25, and .004" for everything else 6.5mm through .45 cal.

The best definition of a throat is that area in the barrel where the rifling are removed to allow the bullet to enter into the "groove diameter" area of the barrel.

The "groove diameter" of the barrel is supposed to be the same diameter as the bullet. But of course in the real world there are tolerance variations and the bullet may be a bit larger than standard or the barrel's groove diameter may be a bit smaller than standard. So when the rifling are cut away to create the "throat" there has to be some allowance for these variations so the bullet can freely enter.

Where this part of the cut stops is generally referred to as the "leade," the angled or tapered ends of the rifling. The "leade angle" may be very long and gradual or more short and abrupt. It is generally agreed that a longer or shallower leade angle is more conducive to best accuracy than one that is more abrupt or closer to the ends of the rifling just being squared off.

The true function of the throat is supposed to be to guide and support the bullet as it undergoes its last forming operation before traveling down the bore. It is in the throat, the leade, and the length of the bullet's full diameter that the rifling engrave into the bullet giving it its final form which will vary only slightly with minor changes in the rifling as the bullet travels down the barrel.

If the nose of the bullet is not centered with the bore, it will enter the rifling out of alignment with the bore. And to the extent the hole the base of the bullet is sitting in is larger than the bullet, the base end of the bullet can deviate in any direction off the axis of the bore making the bullet distorted and out of balance as it spins going through the air sometimes at rates well over 200,000 rpm.

Only a closely fitted steel cylinder with of a significant length and aligned with the bore can effectively guide and support the bullet as it goes into the rifling.

The thin brass case neck cannot force the much stiffer bullet into alignment with the bore, especially when there is anywhere from about .003" to .010" clearance between the loaded case neck and the chamber neck. To think the case neck alone can do this is delusional foolishness.

And to think that either an overly large throat diameter or a throat that is cone shaped from end to end can align the bullet with the bore likewise is delusional foolishness.

SAAMI (the Sporting Arms and Ammunition Manufacturers Institute, that governing body responsible for the designs and dimensions in your chambers) specifications call for diameters for those throat designs that are cylindrical to be commonly anywhere from .001" to .005" larger than bullet size, which has been proven over and over again to undermine accuracy.

In the final analysis, the chamber throat should be viewed nearly on par with the ultra precision forming dies used by bullet manufacturers to produce that "perfect bullet" that will fly to the same point of impact each time you pull the trigger. It is in the throat, once again, that the bullet takes on its final form. If it goes into the rifling canted and thus distorted, it will be out of balance, the base out of square with the shank of the bullet, and cannot fly as "true" as one that is not distorted.

The sad fact is that a big percentage of both factory chambers and those cut by supposedly "custom" gun shops very commonly have throats that are too large in diameter, misaligned with the bore, and/or shaped such that they can do nothing to effectively align the bullet with the bore. This then makes any given barrel problematic and usually ends up in undue time and money spent "working up a load" to compensate for serious deficiencies in the chamber..... deficiencies that there is simply no excuse for.

Note that not one word has been said about throat length or overall cartridge length so far. The distance the bullet moves before coming into contact with the rifling IS a factor, yes, but it is a much, much smaller factor than the much neglected matter of shape and diameter of the throat.

The purpose of this page is to first show what is in factory chambers in particular and then to show what the chamber should look like for accuracy. Chamber casts of both factory and Bellm custom chambers will continue to be added.

Interpreting the casts.

One can obviously see where the rifling start and in many instances there is a bullet positioned where it would first contact the rifling, thus giving a length reference.

In regard to throat alignment with the bore we have examples posted showing misalignment very graphically.

Remember that groove diameters vary and can be either the same diameter as the bullet, smaller than the bullet, or larger than the bullet.
Rarely do you find throat diameters equal to bullet diameters. Typically if there even is a true throat, it will be from .001" to .005" LARGER THAN BULLET DIAMETER!


I ask you, What is a "Shot Out" barrel?
"Shot out" means a lot of wear in the throat...... meaning, the throat diameter has INCREASED due to wear.

So what is a throat cut too large in the first place?

It is essentially a throat that is "shot out" to varying degrees BEFORE THE FIRST ROUND IS EVEN FIRED!

THINK!

If you do a chamber cast of a "shot out throat", worn from thousands of rounds of shooting, it strongly resembles the shape and dimensions of the SAAMI forcing cone chambers now used for all rimmed, straight wall revolver types of cartridges!

In the closed breech guns, there is absolutely NO sane reason for the SAAMI forcing cone chamber, which is the same as a shotgun chamber!
You are shooting BULLETS, not shot!
Bullets have to stay balanced in flight.

Yet this is what "the industry" is defrauding you with when you buy a factory production barrel or a custom barrel chambered by a nit wit company too dumb to realize the difference or locked into SAAMI by their insurance company!

!!!!!!!!!!!!!!
Wake up! Think!

Where the throat ends tells a BIG story.

Bullet diameter, groove diameter, and throat diameter relationships and what they tell.
  • Throat diameter larger than the groove diameter leaves a connecting line between the ends of the rifling at the forward end of the cut where the full diameter of the throat reamer ends and the leade angle on the ends of the rifling continue forward.
  • If such a line is visible, it must be perfectly uniform in appearance all the way around for the throat cut to be centered with the bore, meaning
  • if the the line connecting the ends of the rifling is irregular or appears more pronounced on one side of the barrel, and fainter or non-existent directly opposite this point, then the throat is simply NOT centered with the bore, and
  • in the condition where the throat is larger than groove diameter, there will be no rifling lines visible back in the throat area.
  • If the groove diameter is larger than the throat diameter, then there will be traces of the rifling in the throat area, and
  • if the throat is centered with the bore, then any traces of the rifling in the throat will appear of the same depth.
  • The degree to which the throat is NOT aligned with the bore, the traces of rifling in the throat area will be deeper or more faint directly opposite each other and in some instances riflings will be cut away completely on one side and extend all the way to the chamber neck opposite that area.
  • On the assumption that the throat diameter is at least equal to bullet diameter, no connecting line between the ends of the rifling and/or traces of rifling in the throat of a barrel with little wear in this area indicates a relative difference between the two and tells one that the groove diameter is larger than bullet diameter and perhaps to some degree even oversize.

.22 Hornet Contender 14"
Very short cone shaped throat, not the best for accuracy.

.221 Rem. Fireball factory throats, long and short variations.

The first set of 4 pix is of a badly misaligned factory throat shown in views #3, #1, #2, #4.
Note how the ends of the rifling are not evenly spaced from the end of the neck. This is due to a bad case of misalignment with the bore.

This is the same cast as above, but note how in this view the ends of the rifling go all the way back to the chamber neck.
#2 looks normal from this view.
#4 you can see again the ends of the rifling are not the same distance from the end of the neck.

.221 Rem. Fireball, short throat in the factory barrel TC replaced the above barrel with.
The new replacement barrel has a very short throat, but it appears uniform all around and pretty well centered.

I would prefer to see a longer throat, one that will guide and support a longer section of the bullet's shank as it is engraved by the rifling.

Such a short throat can always be extended with a precision aligned, minimum diameter throat that normally improves accuracy.

.222 Rem. Encore

.223 Rem. Factory Contender Chamber, Short Throat
This throat was lengthened .055" to permit seating bullets out and to give a section of throat that is matched to bullet diameter. Note that between where the chamber neck ends and the rifling starts, the diameter cut by the throat section of the reamer is substantially larger than groove diameter, thus most likely larger than bullet diameter by a significant amount.

This cast was an old cast, thus only a relative difference in diameter between the groove area and the throat area can be established.

Normally military ammo requires a longer throat, and while TC does not warrant their barrels to shoot military ammo, lengthening the throat makes it possible to shoot military ammo quite well. Without an adequate throat length, military ammo produces too much pressure.

A classic case of bad throat alignment with the bore.

7mm JDJ #2
Note the difference in distance of the ends of the rifling from the end of the neck.

View from the opposite side, again note the distances from the ends of the rifling to the end of the neck and how they are angled.

.30/30 factory chamber with a proper, true throat.
.30/30 factory Super 16 barrel.
This is what a throat should look like, and if it is aligned with the bore gives the best probability of good accuracy.

Over the years TC factory .30/30 chambers have been cut in a number of configurations, some excellent like this one, while some vintages had NO throat at all.

Note that in front of the chamber neck the rifling are cut away for a significant distance giving a steel cylinder to positively align and support the shank of the bullet as it enters into the rifling.

Accuracy from this type of .30/30 throat is usually quite good.

.30/30 factory chamber with NO throat!
.30/30 Remington factory ammo and chamber cast of a recent vintage TC factory S-14 barrel.

Older vintages of factory .30/30 chambers like the one above had a true throat in them, but since the early 90's, TC has gone to a chamber that has NO throat in it. You can see the chamber neck is about .050" longer than the case of this Remington factory round, then there is about another .050" taken up by a an abrupt cone that runs right to the ends of the rifling. There is NO further leade on the ends of the rifling and NO cylindrical area that is anywhere near bullet diameter.

About .050" in front of the cannelure on the bullet you can see where its .308" diameter shank ends, and it is at this point approximately that the bullet hits the abrupt ends of the rifling. There is nothing to align the shank of the bullet with the bore except for the case neck.

Controlled tests have shown dramatically that this type of chamber normally produces, in the words of the one doing the test, "dismal accuracy" with groups averaging from 2" to 4."

This factory round in this particular barrel, by the way, fell .005" BELOW the end of the barrel. The barrel-to-frame gap reported by the owner was .004." .005" plus .004" gives a total headspace of .009," .003" over maximum.

.30/30 can be very, very accurate, but not when the accuracy potential is seriously undermined by this no-throat chamber design. Choosing such a design is a waste of your money and an insult to the American gun making trade in my studied opinion. Mike Bellm

Reference the .30/30 chambers with no throats and the effects of cutting a throat into the barrel, even without a full rechambering.

This is what the customer reports!

Mike, in today's newsletter you mentioned the factory 30-30's with no throat. A couple of years ago I sent you my G2 for trigger work. On your own initiative you made a cast that confirmed the worst case throat scenario, and subsequently used that cast as an example that is still pictured on your web site. You did correct my G2 and it has since then been a pleasure to shoot.

Not content with the 100-150 yard limitation with which the 30-30's have been labeled, I've been working on a long range load. This week I shot extensively from 100-300 yards. The bottom line is my being able to hit, more often than not, a three inch target at any distance out to 300 yards.

I use the Burris 3-12X32 scope with the Ballistic Plex reticle and am assembling range cards for both the 12X and 10X settings.

My purpose in writing is to confirm the excellence of the throating you did for me; with anything less I would not be able to post even marginally acceptable groups at these extended distances.

FYI, the load is 35.0 gns IMR 3031, the Nosler 150 gn Ballistic Tip bullet and the CCI 200 primer. Muzzle velocity is roughly 2150 FPS. Shooting into wet newsprint at 120 yards, with a reduced load that simulated 300 yards, the Nosler expanded 50% and retained 50% of its initial weight.

(Please feel free to post this testimonial if it makes a point you want to re-emphasize.)

Lloyd

Current style TC factory .357 Magnum Contender 12" chamber, what I refer to as a "toilet bowl" for a throat.

Note how far a bullet is out of the case before it even touches the rifling and that there is NO cylindrical section to guide and support the shank of the bullet as it is engraved by the rifling. The rear of the forcing cone is the same diameter as the chamber neck and tapers down to bore size, the distance across the tops of the rifling, over a total distance of about .4."

Same barrel as above rechambered to .357 Max. with a true throat.


Note where the chamber neck ends at the case mouth compared to the above cast where there is no visible break at the case mouth. There is some of the original cone remaining, about .1" long, but there is a distinct step down at the case mouth.

Note that for shooting cast lead bullets there SHOULD be a short cone section. If there is no short cone, lead tends to expand into the void between the end of the case neck and the start of the throat with the result being a ring of lead sheared off the bullet as it passes into the throat.

Forward of where the original cone ended, a throat is extended forward .2."


Click here for diagrams of .357 Mag and .357 Max. throats

.357 Mag. older 10 inch barrel
Short conical throat for less bullet jump, but no real support of the shank of the bullet.

.44 Mag. factory chamber

Mid 90's vintage, 14" Contender. Bullet is positioned where it would first contact the rifling. Note the distance from the case neck to the base of the bullet.

.45 Win. Mag. 14" barrel
Note NO throat whatsoever.

.45/70 Encore Katahdin
.45/70 factory chamber, no throat



The factory chamber has no throat, only an approx. 45 degree chamfer at the end of the chamber. The rifling go all the way back almost to the mouth of the case making it necessary to seat bullets extremely deep in the case.

Factory ammo has bullets seated deeply enough to work with the TC factory chamber, but as noted here, component bullets such as this have to be seated below the cannelure.

The point forward of the cannelure on this 350 gr. Hornady bullet where the full .458" diameter shank ends is lined up with the ends of the rifling. Note how deeply this bullet must be seated into the case neck and that there is no cylindrical throat section to support the shank of the bullet as it is engraved by the rifling.


.45/70 rethroated
.45/70 rethroated with throat cut in .300" long.


Same 350 gr. Hornady bullet is positioned where it would contact the rifling. Note there is an ample amount of bullet shank that would still be in the case where there is a significantly long cylindrical section at just under .459" diameter to guide and support the shank of the bullet as the rifling engrave the bullet.

A longer 405 gr. bullet would have more shank in the case, a 300 gr. slightly less while both can still be seated to the rifling.

The length of throat cut can be tailored toward lighter or heavier bullets or even for specific bullets seated to a specific depth based on dummy rounds the customer supplies.

In the photo above, for example, one could hold the throat length back about .050" and be closer to optimum with 300 gr. bullets, or the throat can be cut longer for 400 to 500 gr. or heavier bullets.

In the Encore, if desired, one can come pretty close to .458 Win. Mag. performance with 500 gr. bullets by seating the bullets out into a substantially longer throat, thus creating the necessary powder capacity. The action itself will handle .458 Win. Mag equivalent loads, however, the thinner .45/70 brass may not last as long as the heavier .458 Win. Mag. brass.

Unlike a bolt action .458 Win. Mag., rounds are not subjected to battering in a magazine box under recoil. Thus bullet neck tension is not an issue, and less bullet shank in the case is not a problem.

Speaking of .458 Win. Mag., of course the .45/70 barrels can be rechambered to .458 Win. Mag. also. I have also done a few .458 Lott chambers in Encore barrels. Either way, your Encore can be properly outfitted and ready when dinosaurs raid your pea patch.


.45/70 throat comparison
Bottom, factory chamber. Top, same chamber with throat extended .3"


Here are the two casts of the same chamber side by side.

.300 Whisper with misaligned throat, TC custom shop
.300 Whisper TC custom shop, offset/misaligned throat.
Note that due to the small capacity of the parent .221 Fireball case, bullets for .300 Whisper are normally seated out of the case neck some distance. As you can see here, the rifling extend all the way back to the end of the chamber neck.

....... rotated approx. 180 degrees
.300 Whisper TC custom shop, view from opposite side of casting.

Here you can see the rifling are cut away, as they SHOULD be normally, so the bullet can extend up into the bore outside of the case neck.

However, since the rifling go all the way to the neck on one side and are cut away on the opposite side, this forces the bullet out of alignement with the bore. It is simply stated, pointed in a different direction! How can you expect decent accuracy from something like this? And TC calls it "custom"!

Also, observing the shrinkage and growth factors of Cerrosafe casting alloy, preliminary measurements indicate the diameter of this throat to be about .311," .003" larger than bullet size, when ideally the difference between throat diameter and bullet size should be about 1/10th of that, .0003" for better support and guidance of the shank of the bullet as it enters into the rifling.

With this degree of "slop" around the bullet shank and the gross misalignment of the throat with the bore, how can one expect such a barrel to be accurate, in spite of the premium price paid for this stainless steel "custom shop" barrel?

The only hope for such a barrel is to cut a longer chamber into this barrel, one that will cut out all of what the factory cut; ie, the neck of the new chamber must extend forward of where the rifling start as indicated by this casting.

A new throat is then cut in the bore ahead of the new neck. Steps must be taken to insure that the new throat is cut centered with the bore, which is what I do routinely.

.454 Casull Factory Throat. Small wonder accuracy is mediocre with this round.
Pic #1, 300 gr. bullet. Note that even a 300 gr. bullet is completely out of the case neck before contacting the end of the cone/start of the rifling. The bullet measures .451" while the area in the barrel at the base of the bullet is about .480" and gives NO alignment of the base of the bullet with the bore.

"Ray" made this contribution to the library showing the existing factory throat and steps he took to compensate for the poor SAAMI chamber design for this round.

Mike
Here are some pics for your throat library. #1 is a 300 gr hp, #2 is a 250 gr hp, #3 is my fix. I started with 400 gr 45-70 .458" sized to .452" seated one to second groove group got better seated second in first groove better yet went a 450 gr lead .458" sized to .452" set on rifling 1.5" group at 50 yds. Originaly it was 3.5"thought you might like to hear about this.

Ray


Pic #2, 250 gr. bullet even further from the case neck.
Compensating for the overly long, cone shaped throat above......
Note that in each instance there is still no guidance or support of the base of the bullet sitting in an overly large diameter area, but longer bullets get the bullet nose closer to the riflings.

If the .454 Casull or any other round is given a true, cylindrical throat that supports the shank of the bullet, superior accuracy results. As it is, the SAAMI designs "the industry" is strapped with is costing you money and not giving you the accuracy you expect...... and deserve.

What can rethroating do for accuracy?

Go from this horribly designed, rough factory throat INaccuracy 4 to 6" groups.....
375 H&H, RL 15 powder, 270 grain Barnes solid copper bullet.

(Unfortunately, I did not do a chamber cast of this one, but it was a mess)

to this one hole group, just by extending the TC factory throat with a minimum diameter, long leade angle, well aligned throat.
Same load, 375 H&H, RL 15 powder, 270 grain Barnes solid copper bullet.

Here is another example, .308 Win. with unacceptable accuracy, rethroated, and shot this group with little to no load workup:

Must read.... LBT Cast Bullet Moulds
Veral Smith is probably the premier authority on cast bullets. He is also one of the very few to make the connection between throat diameter and accuracy. His focus is cast lead, but what he says about cast lead bullets and throats applies to jacketed bullets as well.

He also supplies how-to information and soft lead slugs for getting a true indication of throat diameter, length, and configuration that is highly useful also for jacketed bullets.

If a gun is properly assembled and still does not shoot satisfactorily, 9 times out of 10 it is due to the throat and the grossly ignored role the throat plays in producing good accuracy.



Fixing a bad throat.

If rechambering to a longer cartridge is an option, the longer chamber cuts out much or all of the original throat and permits cutting a proper throat. The result is "better than new" accuracy.


.300 Win Mag is a prime example of a very cost effective, simple way to salvage a bad barrel.
1) Factory throats are quite short.
2) .300 Win. Mag. is somewhat hard on throats, and
3) occasionally I see really chewed up, oversized throat diameters from the factories, as was the case with this barrel.
New chamber is designated .300 Win. Mag. 2.850"

Just extending the factory .300 Win. Mag. throat improves accuracy substantially.

But another option is to extend the chamber neck while leaving the chamber body original, run a .300 H&H case into a .300 Win. Mag.size die, and create a .300 Win. Mag. case that has a .2" longer neck.

This cuts out nearly all of the factory throat and permits a new, proper throat to be cut.

The barrel shown above was traded in as an example to demonstrate what such a resulting cartridge looks like.

Every so often a factory throat is cut WAY oversize, sometimes really chewed up from chips, dull reamer, or ??? Such was the case with this barrel.

I recall a .300 Win. Mag some years back that had been shot a lot, such that, as I recall, the washed out throat diameter tapered down from about .320" in front of the chamber neck. This was on a bolt action. I set the barrel back, rechambered it, and it readily shot into 5/8" at 100 yards.

So whether to correct a factory throat or revitalize a worn throat, keep this simple fix in mind for your own barrels.

Barrels that have been shot a lot are smoothed up, broken in, and when given a new, proper throat shoot better than new.

In the example here, there is no change to the chamber body. It still shoots standard .300 Win. Mag., but there is a .2" jump before the bullet enters the throat.

Cases are simple to make. Just run a .300 H&H case into your .300 Win. Mag. size die to push the shoulder back. Trim to 2.840" if needed, load and shoot just as you would with any round.

With the increased neck length and thus chamber volume, you can either moderate force on the frame using standard .300 Win. Mag loading data or up the charge a few grains, of course watching pressure signs. However, I lean toward dropping the pressure a tad to make life easier on the frame.

Another point......
A longer case neck tends to make life easier on the barrel's throat by taking much of the wear caused by solid unburned powder crammed down the small hole. Getting off into more theory, one that was popularized a bit some years back was the concept of the angle of the shoulder should converge inside the case neck. The neck on the .300 Win. Mag. is short to start with, so if you adhere to the theories about longer necks being better, here you go.

I've salvaged a few .223 Rem. barrels in the same manner by extending the neck for the longer .222 Rem. Mag. case while leaving the case body original.

In both instances, you use the standard dies. Only the neck length is changed.

Here is a quote from a customer/serious shooter:
"I sent a Contender .223 barrel for you to extend the throat. After you cut the new throat it went from being woefully inaccurate to the most accurate barrel I owned at the time and I shot my smallest group ever, which stood for many years until I took up benchrest competition. This chambering which you called 223x1.85, I have recently found out once won the Super Shoot (I forgot to ask what year). It was shot by Larry Baggett and was called 222.7."


 

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