Many chambers are cut deeper than ideal and to get good case life it is
necessary to blow the shoulders forward to fill the distance from the chamber
shoulder to the breech face. Once the shoulders are correctly fire formed
forward without stretching the case, you will get much better case life. And of
course there are wildcat cartridges whose shoulders must be fire formed forward.
To blow shoulders forward, I often hear fellows talking about seating bullets
out into the rifling. However, this approach is marginal at best. Most of the
time the firing pin merely drives the case forward over the bullet. This leaves
a gap between the case head and breech, i.e., headspace, and the cases get
stretched anyway. Perhaps not as badly, but stretched none the less.
Even with chambers cut to the correct depth, there is a certain amount of
collapse at the shoulder when the firing pin hits the primer. Take the
extractors out of a few rimless barrels, fire cases with no bullet or powder,
and you'll see what I mean.
Many barrels will not even fire standard ammo without the extractor in place!
Again, try it for yourself.
In test firing a 7mm TCU barrel today, I pulled out some rounds from years ago
that did not fire due to test firing without the extractor installed and the
shoulders collapsing. Annoyed about the whole idea, I got a brain storm.
The 7mm TCU has a very limited capacity, and many charges for it are compressed
under normal circumstances anyway. Added to that, I held the throat to my
standard length of 2/3 of bullet diameter, and it was too short for the 145 gr.
bullets I pulled out.
I pulled the bullets on the rounds that had not fired, re-primed the cases, and
charged them with 24 gr. of a stick powder that filled the case nearly to the
top. This prevented seating the bullet deep enough to chamber in the barrel, so
I pulled these bullets and backed the charge off a about a grain, and tried
again. Cases still stuck out of the end of the barrel quite a bit, but the
bullets were down snug on the powder. So, I thought, the charge is low enough,
the powder slow enough, what will it hurt if I can compress the powder enough to
give adequate support against the blow of the firing pin?
It works beautifully! Both cases fired just fine and blew the shoulders forward
without any sign of case stretching at all... shoulders were blown forward at
least .020." I.e.., that is how far the empty cases originally dropped below the
end of the barrel when chambered.
So what is the application? How do you use this info in actual practice?
Let's say you have a factory .22 Hornet which is notorious for stretching new
cases due to the chambers being deeper than they should be, or the case rims too
Pick a powder slower than anything you can generate normal pressures with. In
this case it might be 4198 for example, or you may have to go to H-322. Start
Low, Work Up until you are certain that a case filled pretty much to the top
cannot produce too much pressure with a bullet long enough to solidly contact
the rifling. Work toward the point where the bullet compresses the powder firmly
enough to support the case against the blow of the firing pin. With the powder
firmly compressed, the firing pin cannot drive the case up over the bullet.
You may have to drop to a still slower or bulkier powder to get the powder
compressed firmly enough the firing pin cannot drive the case forward, but using
care and caution, I see no reason why this cannot be done with virtually any
case needing the shoulder fire formed forward.
end up using 5010 or 870 to do it in some instances without raising pressures
too much, but it will work.
This may be a better method yet than pistol powder and Cream of Wheat in a lubed
the first time I have tried this, and it worked beautifully. It is a much better
approach than trying to rely just on neck tension on the bullets to do the job.