Pressure and subsonic loads with fast powders

[st1650]

Hi,

I've been handloading subsonics for the better part of the last 15 years (300 whisper, 300BLK, 308win). Couple of my friends and I want to get into the 450BM to load the new-ish Hornady 395 SubX.

Obviously most load data and factory ammo are meant to cycle ARs which means generally slower burn rate, dirtier, higher SD/ES spread due to low case fill.

I'm planning to load using VV Tinstar because its a very fast powder that is more dense that other powders such as N105, LilGun, 4227, etc but doesn't overfill the case like Trailboss, which would probably be perfect for the 250-300gr XTPs.

Now I'm running theoretical numbers with QuickLoad and it seems like I'm near max pressure with a case fill of 80% at around 1050FPS. My question are:

1- Since we're all going with bolt guns and not limited by the AR platform magazine length, would it be a good idea to increase the freebore a tad in order to load these long Hornady SubX maybe at 2.28 therefore lowering the pressure a bit ?
2- I've never handloaded for a rifle straight wall cartridge, but is 39K PSI the true upper limit for 450 bushmaster ? Seems way low for a bolt gun with that kind of case head . So say using CCI 450s primers, new Starline brass and a slightly longer chamber, do we get a bit more leeway on the pressure or 39K is more or less the hard wall ? Was the 39k pressure originally determined by the AR15 platform ?
3- Do you have to crimp at the cannelure ? With these long Hornady 395, seems like if you seat and cramp them at the cannelure, you have to load them at approx 2.220in. Would it be less accurate to seat them longer and not crimp on the cannelure ? I've shot a lot of SMK 220/Lapua 200fmjbt in subsonic and never cared about cannelure position and I would be able to shoot 1-1.25moa at 200 yards.
4- Accuracy. Obviously, subsonic and long range accuracy is a bit of a challenge but in both my 308win sub and 300blk, I was able to get my loads down to single digit ES and shoot a 6in gong at 325yards with ease. G1 BCs don't really matter in the transonic/subsonic world so I was wondering if I can expect similar accuracy (at least 2-3in at 200 yards, with mostly vertical spread).

[Al in Mi]

The 450 has .200 freebore so if your loading for a bolt gun, seat a dummy round to the lands and then I'd back off about .030 and work my load fron there.

That 36k max pressure is for AR's, and I've ran some pretty hot loads thru my custom 700 and my ruger #1. You know the drill, start low and work up.

I have some of those 390's to try, hopefully soon.

[Al in Mi]

Just seated in my #1, 2.325ish to lands.

[st1650]

Very nice, thanks for the info.

What kind of crimp do you do on your bolt gun loads ? Planning to use the Hornady Custom 4 dies kit.

Also what kind of mags do you use for your R700 ? Any luck with AICS 308 mags or is it better to use 6BR magazines due to the shorter coal with the spacer. I'm not worried too much about the feedlips because it's easy to open them to fit the case but more about the relationship between when the case is stripped from the lips to the barrel. I might do a bit of a chamfer similar to how Accuracy Intl does it to help it feed smoother.



Planning 2-3 different builds, couple would be on Tikka actions, 1 might be a SBR on a Remington700/Defiance Action and one would be on a Desert Tech Covert with a 16in 1.250 straight contour. I know a few people got the Desert Tech to feed perfectly with only minimal modification to the mag.

Was originally considering a 458/2in but I think 450BM is a better caliber if those SubX 390 bullets can deliver accuracy and expansion.

[Al in Mi]

I just taper crimp with the Hornady die but have a Lee crimp die to try.

The 700 I'm feeding right from the factory BDL box. No issues with FTX or 275gr Barnes. The Barnes are 2.420ish oal.

[coyote wacker]

The 458x2" is a great caliber and much better than the 450 Bushmaster....

450 BM was designed first for a AR-15 that's were you get the lower pressure and the cartridge case length of 1.7".....if it was loaded for bolt guns only no problem 60'000 psi at that pressure and a bolt gun only thing needed is bullets designed for that pressure and speed.....

458x2" is in a hole different class compared to the 450 BM with proper bullets nothing walking this Earth would be too much for this caliber....only problem is in Michigan and some other States is the case is too long to use for hunting.....

[Al in Mi]

The 458x2" is a great caliber and much better than the 450 Bushmaster....

450 BM was designed first for a AR-15 that's were you get the lower pressure and the cartridge case length of 1.7".....if it was loaded for bolt guns only no problem 60'000 psi at that pressure and a bolt gun only thing needed is bullets designed for that pressure and speed.....

458x2" is in a hole different class compared to the 450 BM with proper bullets nothing walking this Earth would be too much for this caliber....only problem is in Michigan and some other States is the case is too long to use for hunting.....

Made me up a 458 x 1.8 on a model 7 few years back, never really wrung it out, so polished up some brass last weekend and gonna try a couple loads hopefully this weekend.