The official Mike Bellm's

Bellm TCs

TC Contender, G2, Encore/ProHunter, & CVA Apex Performance Center


Here you see the riflings do not start on the cast until about 5/16" from the end of the chamber neck!
Here you see rifling coming back almost to the end of the chamber neck.

The bottom line is that chamber throats are something normally kept hush, hush in the firearms trade in general. Everyone talks about overall cartridge length, seating depth, freebore, and all the various words skirting around what is actually inside the barrel and what its function is.

No one is talking about throat diameter or throat alignment with the bore, yet this is the single most important aspect of the chamber.

Note that the casts have been highlighted to give clearer contrast between the lands and grooves in the cast.
Here is the main cause of chamber misalignment that is often quite severe in Contender barrels made during the first approximately 25 years of production of "drill press" chambers......25 years for "America's Master Gunmaker" to figure out a very basic machining principle that is so vital to accuracy.
It was shortly after this picture was printed in the TC Custom Shop catalog that apparently pressure was brought to bear on Thompson Center to change their method of chambering barrels.

It was salted away but recently surfaced after the move to Olathe, Colorado.

The picture is grainy, but the middle right picture is of a barrel held vertically in a fixture with a chamber reamer held in a drill chuck poised over it.

What is wrong with this picture?
Beneath the reamer is a barrel held vertically in a fixture that references off the outside diameter of the barrel.

1) Any change in diameter from standard sets the bore off axis from the reamer.

2 ) There is at least .005" variation in barrel diameters, sometimes more.

3) All bores have some curvature in them as drilled, warpage is induced turning the barrel blank down to size, and welding the lug on induces warpage in the bore so even if the bore is centered at the extreme breech end. At the area where the throat is, it is generally way off center.

4) Held in a drill chuck the reamer cannot follow the bore, thus

5) unless by chance a chamber was centered with the bore, you will find most chambers especially from the earlier "drill press days" dramatically misaligned with the bore as typified by the chamber casts show above.

6) When rechambering these barrels the warpage must be taken into account and corrected for. Most "gunsmiths" and many "in the trade" are not even aware of this as demonstrated by the most notable of notables who is the subject of the above casts.

The chamber casts above are from a 7mm TCU Contender barrel rechambered by one of the biggest names in TC after market barrels. Someone paid good money for this and yet as this is posted, has had no satisfaction from the shop that did it.

This is how it was handled by the shop that did the work. Their reply:

"One problem with rechambering is that the reamer will follow whatever hole is there. From the looks of it the bore is oversize or the pilot would not have let it that much off center and would have scored the bore. This is the worst I've ever seen. Seeing 2--300 FPS difference in velocity in T/C 7 MM barrels is normal. Seems to be interior dimension problem.
We have a 7 MM throater and about the only thing we could do is run the throater in it and see if that would help. (no charge)

Let me know what bullets and loads you were using--maybe a change would help."

Point One: It is an easily proved fact that in the real world reamers do NOT closely follow the bore.

Several subpoints are in order.

1) Bore sizes will vary by AT LEAST .001," and it is not uncommon to see bore sizes .002" to .003" on either side of standard. And, typically, reamer pilots are made at least .001" smaller than standard bore size. So if you have a .275" reamer pilot diameter for the 7mm barrel above, and a .277" bore, there is at least .001" the reamer can wander in any direction. The rifling, btw, are only .004" high in 7mm. .276" standard bore diameter, .284" groove diameter. .008" difference in diameter divided by 2 equals .004" rifling height.

2) Heat applied to one side of the barrel welding the lug on does induce warpage in the bore.

3) Even IF a tightly fitted pilot or pilot bushing is used, hard or not, reamer pilots will still bend.

4) The barrel above was originally a 7mm TCU, very likely from the days when TC poked chambers at bores with a drill press. Many of these chambers are badly misaligned with the bore to start with, and unless the resulting runout is corrected first, the reamer will tend to follow the existing chamber.

5) It is CRITICAL that the bore be running true in the lathe and that the throat be cut as a separate, final operation independent of the body of the chamber. It is the only way to assure the most precise alignment with the bore.


(I am not talking out of both sides of my mouth. Reamers will try to follow the existing hole, yes, but if there is substantial runout, the pilot cannot overcome this. Ie, it is the runout in the original chamber that is much of the problem.)

Point Two: Oversize bore or not, there is no excuse for misalignment of the throat with the bore..... and then not stepping up to the plate to correct it, either with a viable rechambering option or with a replacement barrel.

Point Three: Creating a longer throat is not the answer.
It will result in a long "freebore," jump to the riflings, while still having a substantially misaligned area in the throat.

Point Four: Throwing a different load at the barrel is not the answer.

The problem should be as obvious as a train wreck. Starting the bullet into the bore cock-eyed and creating an out of balance bullet is not going to be corrected by a different load.

Point Five: We all make mistakes, but over the years should be learning better ways to do things.

.221 Fireball TC factory chamber. Photos provided by the same customer.
Note that the rifling come all the way back to the chamber neck.

In this photo, you can see that the rifling at the top of the cast start at about 1/8" from the chamber neck.

This shot shows more of the angular view of where the rifling start and stop.

The .221 Fireball barrel is to be sent back to TC.

What do you do?

First, become aware of the importance of throat alignment and do not suppose that just because it came from the factory or it came from a high priced shop that it is right. Any fool can buy a reamer and poke it at a bore. Never assume that a big name and a high price is any assurance the chamber and alignment are right. I have seen examples as bad as the above even done by some of the most renowned benchrest gusmiths.

You can look into a throat and see much of the misalignment once you know what you are looking for. And as you can see, a chamber cast will confirm alignment or misalignment.

Don't take it. Pass the error back to the originator. If you get no satisfaction, I strongly recommend a small claims court filing. Just the mention of which has opened the ears of TC's personnel and obtained their fullest cooperation.

If you do nothing, you will continue to be taken advantage of. If you hold their feet to the fire, they will begin to see the light, hopefully respond with more responsible chamber work to start with, and be more responsive when there is a problem.

The sword cuts both ways. If you have a barrel I did with throat misalignment, please bring it to my attention also, because based on what I have learned over the years, I am sure I have some chamber throats out there with misalignment sufficient to undermine accuracy also. But during the last 15 years of both chambering and rechambering, I have made ever improving strides toward perfecting the throats I cut.

Favorable outcome of the .221 Fireball sent back to TC:

The owner of the .221 barrel sent pix of the chamber casts to TC along with the barrel, and they DID replace it with a 14" barrel.

Being an informed customer DOES make a difference. I may be too optimistic, but I think it is far better for everyone if vendors are made aware of errors and will hopefully raise their standards instead of just rolling their warranty costs back into the retail selling price.

Here is a cast from the new replacement barrel

Alignment appears to be much, much better.

Note the difference in the length of the throat. The replacement barrel has a much shorter throat. I prefer to see a longer throat like the original that will guide and support a longer section of the bullet's shank as it enters the rifling.

The clearances in the neck area cannot give positive alignment and support of the bullet shank, so in the replacement barrel only a very short section of the bullet shank is supported.


  eCommerce solution by
  Nibbles Knot Web Authoring

  Web Hosting by


All Images and information contained on this site are the intellectual property of Bellm TC's.