303Guy
03-28-2013, 11:41 PM
65714
Here we have two boolits of the same shape and weight fired with the same load into the same catch medium. The difference between them is the alloy strength and hardness. If one looks closely one can see the top one has what looks like a narrower middle. Actually it is not narrower than the front but simply has no knurling. The boolit has a two diameter form with a taper step. Note the rifling impressions in both boolits. The bottom boolit has rifling impression almost up to the nose while the top boolit has rifling impressions to the middle of the step down taper.
Here's what's going on. The top boolit has not undergone any bump up while the bottom boolit has bumped up to just before the nose. That represents the optimum alloy strength for the pressure generated by the charge. The first boolit can be expected to have poor bore alignment and hence poor accuracy. The bottom boolit on the other hand has had just sufficient bump up to support it in the bore and align it. This is what I mean by balancing the alloy strength to chamber pressure. But what about too soft an alloy?
The nose gets bumped up as in 'nose slump'. So the optimum alloy strength or hardness will allow the bullet to obturate or bump up fully without nose slump. (Forgive the oversize photo - I'm struggling with the new look, impossible to work with photobucket!)
http://i388.photobucket.com/albums/oo327/303Guy/Riflingskidnoseslump002.jpg
These were paper patched boolits which is why the noses were under bore diameter.
Here we have two boolits of the same shape and weight fired with the same load into the same catch medium. The difference between them is the alloy strength and hardness. If one looks closely one can see the top one has what looks like a narrower middle. Actually it is not narrower than the front but simply has no knurling. The boolit has a two diameter form with a taper step. Note the rifling impressions in both boolits. The bottom boolit has rifling impression almost up to the nose while the top boolit has rifling impressions to the middle of the step down taper.
Here's what's going on. The top boolit has not undergone any bump up while the bottom boolit has bumped up to just before the nose. That represents the optimum alloy strength for the pressure generated by the charge. The first boolit can be expected to have poor bore alignment and hence poor accuracy. The bottom boolit on the other hand has had just sufficient bump up to support it in the bore and align it. This is what I mean by balancing the alloy strength to chamber pressure. But what about too soft an alloy?
The nose gets bumped up as in 'nose slump'. So the optimum alloy strength or hardness will allow the bullet to obturate or bump up fully without nose slump. (Forgive the oversize photo - I'm struggling with the new look, impossible to work with photobucket!)
http://i388.photobucket.com/albums/oo327/303Guy/Riflingskidnoseslump002.jpg
These were paper patched boolits which is why the noses were under bore diameter.