Moderator: MudBug
Hoot wrote:IMHO, with a short carbine (16") length barrel, you may be seeing the plateau because you're throwing the additional powder out the the muzzle before it can generate additional velocity. Otherwise there is no explanation given your choice of primer. I don't know another member who uses such a severe crimp. With a taper crimp, there comes a point where the effect reverses due to resizing down a majority of the bullet's bearing surface, resulting in less bore pressure. I've never crimped less than .474 measured precisely at the lip with a micrometer. Even then, it was down into a driving band groove on Barnes bullets. The further down the case you measure, the thicker the case wall gets. That's why it should be measured carefully, as close to the mouth as reasonably possible. Gauging the taper crimp resultant diameter, further down the case, contributes toward squeezing the bullet smaller in diameter. The taper crimp doesn't just taper at the very end of the case. The inside of the taper crimp die is not curved, its slanted. The further up into the die that you push the cartridge the further down the case that slant is imparted. I tried saying this several ways since I don't know your experience with internal ballistics. Not trying to belabor the point.
With the 250 FTX you might want to limit your crimp diameter to no less than .475-.476 and if you're crimping the case mouth down into the bullet's cannelure with the 225 FTX, then you can go a little further, say .474-.475.
That's something you can experiment with since you're up against a conundrum. Admittedly, its grasping at straws but at least its something to try. Lastly, my experience with Lil Gun vs W296 has been that I saw plateauing more with W296 than Lil Gun, but that's just my experience and admittedly I was using a 1:24 twist 20" barrel on an AR platform, which obviously is a horse of a different color than a bolt action.
Hoot
Hoot wrote:No need to respond right away. I know how it is at work. I've mic'd Hornady factory 250gr and they've been between .475-.476 at the mouth. Historically, the velocity SD yielded by the 450b taper crimp, has been higher than I would ever tolerate with a bottleneck caliber. A lot higher. When they first came out, the Remington Accu-Tip 450b loads I measured, had no taper crimp at all and they flew like it. Heavier crimps do tighten up that SD but this caliber takes such a rainbow trajectory that their 100 yd groups are still amazingly tight off a BR setup with modest taper crimps. The velocities you observed are not unheard of with a 16" barrel and the SAAMI spec'd COL of 2.25. I expect that there will came a day when there are two sets of specs. One for the original AR platform and one for Bolt Action/Single Shot rigs. For now, with your 16" barrel, I'd stick with Lil Gun's pressure curve for bullets <= 275gr and W296/H110 for heavier ones, but then that's just one opinion.
Hoot
Kawi-vn750 wrote:Had to check my crimp measurements when I got home. I’m getting .473 on the factory loads. But I did notice that there is a bit of play in my calipers which I’m sure has skewed my measurements. Time to pony up for a better set( if the boss lady approves that is). With that said, according to my wonky calipers I am crimping .002 more than factory which is all the further I care to go considering the round head spaces on the mouth. As far as powders go, i think i will try the h110 on the next batch. Curious if anyone has any luck or tried Enforcer or Accurate No. 9? Or maybe N120?
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