Bellm ‘Triad’ Cartridges

Bellm cartridges based on the .308 Winchester case body and .444 Marlin brass to both correct factory chamber misalignment and increase performance from the Thompson Center Contender and G2 firearms using common, standard reloading dies for easy, economical case forming and loading.

Custom chambers without the cost of custom dies!

Bellm cartridges in 7mm, .308, and .358 calibers for the Thompson Center Contender & G2 firearms with thoroughly tested and proven reloading data.

.308 Bellm, 7mm Bellm, .358 Bellm, load data, Thompson Center Contender, Thompson Center G2, rechambering

Encore No. 2 versions.

1) Use standard size die and have me open the chamber enough to be compatible with the larger dimensions of standard dies. (This may leave a more prominent bulge at the case web, and if it is going to bulge too much, I will decline opening it that far.  In that case you will have to select between options 2) and 3) below.

2) Use a small base size die for virtually no visible bulge at the web and a close case to chamber fit.

3) Use standard size die in conjunction with a .444 Marlin size die for the Advanced sizing method which leaves virtually no visible bulge at the web and allows for a quasi neck sizing operation, thus using the .444 Marlin size die only when cases have been expanded enough at the web to require running cases through the .444 Marlin size die to reduce the web diameter to near new brass diameter.

Properly adjust the size die:

Both formed and resized cases should always enter freely and protrude no more than the actual (measured with a feeler guage) distance between the end of the barrel and the breech face. The size die should be adjusted in small increments until the case head protrudes just short of this amount. On most barrels and frames, the case head should protrude no more than .002.” The case head must never come to a point below the end of the barrel. This paragraph applies to all bottle neck cases fired in break-open guns. Never attempt to “Neck size” either with neck size dies or by partial full length resizing. You may get away with it in light loads, but will incur extraction problems, misfires, and poor accuracy at the performance potentials of these cartridges. You must “bump” the shoulders back.

Advanced Approach to chamber and size dies

I prefer to have the user’s size die in hand when cutting the chamber for these cartridges in particular. The reason is that I like to keep the chamber mouth close to new brass dimensions, and many of the .308 Win -type dies will not size any smaller than about .470″ at the web. This means having to open the chamber up to the point that it produces bulgy looking swollen case heads in order for the chamber to be larger than the size die.

It recently dawned on me that most of the .444 Marlin size dies will size smaller than most all .308 Win -type size dies, back to within just a thousandth or two of new brass dimension of .465.”

Using a conventional .308 Win -type size die in conjunction with a chamber mouth that gives fired case web dimensions around .467-.468″, one can both size the case neck and bump the shoulder back while maintaining both a correct sized shoulder diameter (ie., no swelling of the shoulder as when trying to push a shoulder back with a neck size die) AND maintaining the fire formed fit at the web area. This is sort of a hybrid neck sizing.

When the web area  becomes expanded by loads hot enough to make it a tight fit in the chamber at this point, simply run cases through a .444 Marlin size die with the decap rod removed. Then the cases will be sized perfectly from shoulder to head.  For 7mm Bellm and .308 Bellm, you can use Lyman .358 Win dies which are about $20 from Midsouth Shooters Supply.  The Lyman .358 size die sizes to about the same diameter as most .444 Marlin dies do, about .467″ or less.



Another option is to purchase small base dies. This lets me cut a close tolerance chamber, and no .444 Marlin size die is required.  However, small base dies are usually priced higher.

Each cartridge is defined as follows:

The standard 7mm-08, .308 Win, or .358 Win chamber is cut .040″ deeper than standard.

Chamber neck is extended for a maximum of 2.215″ overall case length.

Cases for each are formed from full length .444 Marlin brass trimmed to standard .444 Marlin trim length, 2.200.” ( 2.215″ max.)

For Info about each, Click the link below

(No. 2 versions of each are available for the Encore.)

No Expensive Custom Dies Required!

Each uses the corresponding 7mm-08 Rem., .308 Win.,  or .358 Win. dies.

“Advanced” approach also uses a .444 Marlin size die body.

Chamber Drawings and Loading Data for The Bellm Triad:

The data referred to above was developed in a highly controlled regimen, fully tested for pressure and accuracy. It was developed in Thompson/Center factory production barrels rechambered by Mike Bellm. As with all loading data, it is presented for use at your own risk. No liability is assumed by Mike Bellm or Don Shearer for any damages or injuries resulting from the use of this data. This data represents performance at pressure levels deemed safe for use in the Contender. While some promoters of their wildcat cartridges advertise velocities 100 fps or more faster than presented here, they also invite you to stretch your frame. If you choose to exceed the loads presented here, do so with the knowledge that you are on the ragged edge of what the Contender will handle. Also, if you stretch the lower part of the lug on your barrel and it becomes loose on your frame, I will not make any replacement of the barrel to you. You are on your own, and I won’t pay for your mistake.

Even though Thompson/Center does replace stretched frames, it is irresponsible to ask them to pay for your desires to hot-rod your loads.

Stay within the levels listed below, and you will get some of the best accuracy you will ever experience with a Contender along with top long range energy and trajectory performance.

All the best, and good shooting………….. Mike Bellm

Summary of Options for Resizing cases:

(In case you are confused)

Please Choose One of the Following Methods for Resizing Cases for the Bellm Cartridge of Your Choice

With each of the 3 options above, please send your stripped size die body or .444 Marlin stripped size die body if you choose to use one.  Or, your .358 Win Lyman size die body.  Confused?  I want the die that will size the smallest at the web to make sure you can size the case webs down.

This lets me check dimensions and assure a good match of dies and chamber. It eliminates possible problems associated with variations in various size die dimensions.  Please DO NOT send the entire size die or die boxes.  It only complicates packing and shipping.  A stripped size die body is much easier to pack with the barrel, and if there is no lock ring or decap stem installed, it is much less obtrusive and much less likely to do damage to the barrel in transit.

Read also from these links:  The Experiment Every Contender Shooter Should Perform and  Chamber Throats 101.

Please Note: 7mm Bellm, .308 Bellm, and .358 Bellm are reserved primarily for Contenders.   For Encores I have “NO. 2″ versions of the 7mm and .308. For .358 I use the .358 JDJ reamer. Standard 7mm-08 Rem., .308 Win, and .358 Win. dies are also used for these rounds, but with their longer case bodies, a .444 Marlin die is needed for all three so as to be able to size the web area. These are relatively new and no data has been prepared for distribution as yet.

The increased case capacity of course means increased velocities over the original chambering, but the main reason for going to the full length .444 Marlin case is to cut out the original factory throat and allow me to cut a minimum diameter, long leade angle throat that IS aligned with the bore.  Thus the No. 2 versions are intended as much or more to correct the barrel for accuracy as they are for increased velocity.


Size dies must be long enough inside to accommodate the full length necked down .444 Marlin case.  All RCBS dies are long enough inside.   Lee and at least some of the Hornady dies are too short inside and won’t work, except for the standard .444 Marlin die which is ok of course.  Experience with Lyman dies has been limited, but so far they have worked perfectly.  Lyman dies are priced more reasonably, especially the .358 Win. dies, and tend to size smaller than RCBS dies.  This makes the Lyman dies preferred for the dimensions I cut chambers to.   At this point I am leaning more toward the Lyman dies until such time as I learn otherwise.  Bottom line, RCBS and Lyman are my recommendation.  Redding is good, but to avoid having to cut a chamber that bulges cases, with Redding you will most likely have to use a .444 Marlin size die to size the case webs down.  For 7mm Bellm and .308 Bellm, the Lyman .358 Win dies can be used in lieu of a .444 Marlin size die.

Starting .444’s through a .375 JDJ or .358 Win. size die first helps reduce losses in subsequent stages of necking down. Worst loss I have experienced is about 8-10 cases per hundred. Starting with .375 JDJ or .358 Win. can reduce losses to nearly 0.

Always start with the .308-type size die backed off about one full turn. Trim cases to something under 2.200.” Take the barrel off the frame, and drop a sized case into the chamber. Then……

A Little About Case Forming:

.358 Bellm requires only a set of .358 Win. dies to size and load.  Lyman .358 Win dies are recommended.  These excellent dies are available from for under $20, as opposed to over $40 for RCBS and Redding dies.

.308 Bellm can be formed with only the .308 Win. size die, so long as the .444 Marlin brass cases are well rounded at the mouth. Sort out those cases with badly dented mouths which will result in a crease in the neck.

7mm Bellm cases must as a minimum be started in a .308 Win. size die. Even if you have to buy a used set at a gunshow for $15, this is cheaper than buying “form dies.”

4) eliminate the cost and delivery time for custom dies,

5) minimize case forming work and cost of dies to form cases (only 7mm Bellm must use more than just the size die to form cases),

6) eliminate having to fire form cases,

7) provide an ample powder capacity for maximum performance with slower powders, while

8) maintaining a sufficiently high loading density with faster powders and lighter bullets,

9) create a chamber that will not fire high pressure rimless ammo such as 7mm-08 Rem., .308  Win. or .358 Win, and has a greater volume so as to reduce the potential hazards of firing  .307 Win, or .356 Win. ammo in the Contender, AND

10) provide maximum accuracy and performance in a highly “user friendly,” sanely priced  package.

a) Typical Bellm chamber based on the .308Win. body easures  .467″  at the  web area,  .457″ at the shoulder for a total taper of just .010.”

b) Contrary to erroneous statements made by magazines as gospel, it is the  minimum body taper  that reduces back thrust, not the shoulder angle as  they  would have you believe.

c) “Improved” cases with 40 degree shoulders nearly always incorporate a  minimum body taper, and it is the minimum body taper, not the  shoulder  angle, that reduces back thrust. The effects of minimum taper can  be very easily demonstrated, especially in any straight wall chamber.

Get the benefits of a stronger case, a rimmed case, a minimum body taper chamber, and greatly improved accuracy and performance with minimal case forming.

No fire forming required!

Purposes behind the design are to:

1) cut a chamber long enough to cut out the misaligned, oversize factory chamber throat and permit me to cut a precision throat of the correct diameter for best accuracy aligned with the bore (.444 is about.160″longer than a .30/30, just enough to make a huge difference in accuracy!),

2) utilize a rimmed case that is stronger than the thinner brass for low pressure 7-30 Waters,  .30/30, and .35 Rem. brass used in the factory chambers, thus permitting good extraction and case life at higher pressures and thus higher performance levels,

3) take advantage of the reduced thrust back to the frame provided by the minimum body taper of .308 Win.-type chamber bodies and the strong .444 Marlin brass,

The Bellm Triad Cartridges

Cartridges based on the .308 Win. case body and .444 Marlin brass for the Thompson/Center Contender <br>Unfortunately, only Remington .444 Marlin brass is acceptable for necked down versions based on .444 Marlin, and as of this date, 12-10-2015, Remington .444 Marlin brass is not being produced, nor is there any forecast from Remington as to when it will be produced again.

Maximum Pressures With .444 Marlin Based Wildcat Cartridges, Click Here