Firearm chamber casts (cerrosafe), what they tell you and what it means for accuracy potential

Chamber Casts on File for Reference

By: Mike Bellm

   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.

     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.

     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.

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 rifling being in the way. Rifling 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 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 function of the throat is 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, effort, 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 noted 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!

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.

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.

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.22 Hornet Contender 14"

.222 Rem. Encore

.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.

.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.

     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.

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.357 Magnum Contender 12"

Same barrel as at left re-chambered to .357 Max. with a true throat at the case mouth.

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"

Note where the chamber neck ends at the case mouth compared to the above cast where there is no visible break. There is some of the original cone remaining, but there is a distinct step down at the case mouth. Forward of where the original cone ended, a throat is extended forward .2"

.357 Mag. older 10 inch barrel

.44 Mag. factory chamber

Short conical throat for less bullet jump, but no real support of the shank of the bullet

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

.45/70 Encore Katahdin

Note: NO throat whatsoever

.45/70 factory chamber, no throat

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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 throat comparison

.45/70 re-throated with throat cut in .300" long.

Here are the two casts of the same chamber side by side. Bottom, factory chamber. Top, same chamber with throat extended .3"

     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 re-chambered to .458 Win. Mag. also. Either way, your Encore can be properly outfitted and ready when dinosaurs raid your pea patch.

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

.300 Whisper with misaligned throat, TC custom shop

Rotated approx. 180 degrees

.300 Whisper TC custom shop, offset/misaligned throat.

.300 Whisper TC custom shop, view from opposite side of casting.

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.

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 alignment with the bore. It is simply stated, pointed in a different direction!

     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; i.e., 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.

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.454 Casull Factory Throat.

.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.

Pic #2, 250 gr. bullet even further from the case neck.

"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 yards. Originally it was 3.5 "thought you might like to hear about this. Ray

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.

On the other hand, 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.

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This article reprinted with permission of Mike Bellm and Bellmtcs.com

Warning: All technical data mentioned, especially handloading, gunsmithing and bullet casting, reflect the limited experience of individuals using specific tools, products, equipment and components under specific conditions and circumstances not necessarily reported in the article or on this web site and over which The Los Angeles Silhouette Club (LASC), this web site or the author has no control. The above has no control over the condition of your firearms or your methods, components, tools, techniques or circumstances and disclaims all and any responsibility for any person using any data mentioned. Always consult recognized reloading manuals.