450Bushmaster.net

Siringo wrote: . . . you are most likely correct from the beginning -- your 1/16 twist is just to fast and your chamber is different (no free bore - correct). Also, since the bore is .451, does this barrel have shallow rifling similar to the 45 ACP? That would account for the stripping -- but if stripping was really the culprit, your throat area would really be leaded up. I never had any barrel leading.

Did the bullet holes look square to the target face? I can't tell from the photos.

Also, the 460 S&W seems to handle the heavy cast bullets from Cast Performance just fine at 1800 fps. However, the S&W revolver has gain twist rifling -- helps prevent the stripping.

Siringo-
The barrel was originally drilled and rifled as a 45 ACP barrel, so it has a 1-in-16" twist and I assume the standard depth of rifling (0.451) and bore diameter, although I've never slugged the bore to find precisely what the dimensions are.

Manson ground the reamer for this chamber to SAAMI specs for the 450 B. Manson has ground the reamers that Bushmaster uses to make the chambers for all their AR 450B rifles, so the chamber should be the same as most everybody's.

Bullet holes all appear square, with no oblong or oval holes as evidence of tipping and instability. This is an important point as evidence that the bullets were in fact spinning fast enough to stabilize; thanks for thinking of this fact that I neglected to mention in the report above.

Also, I can see no obvious build up of lead in the throat or in the grooves at the throat. It may well be that the bullets are not stripping. So, whatever is causing the problem at 1900+ fps does not appear be as large a factor at 1270 fps. I'll not discount any reasonable hypothesis to explain this.

--Bob

Well -- I am at a total loss. Over stabilized??

What are you using as casting material? lead, wheel weight, tin antimony, or something different for this project? Just trying to get the most information out of this undertaking.

2zero6 wrote:What are you using as casting material? lead, wheel weight, tin antimony, or something different for this project?

2zero6-
Three of the four bullets used are commercial bullets from Cast Performance. All I know about their properties is what I read on their web site. Here's a link: http://www.castperformance.com/StoreFront.bok.

Here's an edited blurb form that web site:

Cast Performance web site wrote:

• Our hunting bullets are cast from LBT style molds.
• Our bullets are cast from virgin alloy and heat-treated to a Brinnel hardness of 18-21. This provides for a hard yet ductile bullet, which will not fragment, or blow-up on the toughest hide or even bone.
• Our bullets do not lead your bore. Our lube has allowed us to shoot our cast bullets in test rifles as fast as 3100 fps.

--Bob

Siringo wrote:Also, since the bore is .451, does this barrel have shallow rifling similar to the 45 ACP? That would account for the stripping -- but if stripping was really the culprit, your throat area would really be leaded up. I never had any barrel leading.

In reply pitted bore wrote:Also, I can see no obvious build up of lead in the throat or in the grooves at the throat. It may well be that the bullets are not stripping.

I need to change my answer. I started to clean the bore, pushing through patches with solvent. The throat felt rough, and many flakes of lead have come out on the patches. The hypothesis that the bullets were stripping at higher velocities is still a viable one.

--Bob

Here is something that I have not taken into consideration -- the small amount of free bore is what dimension? .452 or .453? Should the bullets be sized to the diameter of the free bore and not the bore itself. Much like one would do in revolver shooting? If the bullet is .452 and the free bore is larger, could gas possibly cut bullet at that point and deposit it in the section of the bore just ahead of the freebore? Now I have questions in two areas -- freebore and gas hole. Both not good for cast bullet accuracy.

Generally cast boolits should be sized to fit the throat, rather than the groove diameter. If gas can get past the boolit in the throat it will lead the bore and accuracy will be poor. I size the boolits I use in my .450B to .453 and get no leading at all. I think the chrome bore may be pretty cast boolit friendly.
BD