Previously posted by Ardent:
My new wildcat, 7mm AIM
When I learned of Berger's then new 180gr 7mm VLD awhile back I immediately liked the bullet, and especially its .684 BC. I knew this was a bullet I wanted to experiment with, and after having gone through 6mm's and 6.5mm's, 7mm was intriguing. I started looking for a case, it had to be high pressure, relatively low capacity, and very efficient. 7mm-08, while I love it, was ruled out as velocities would be abismal with the 180gr bullet. .284 Win was considered, and eventually ruled out as (and yes this is odd) having slightly too much capacity. As well, while an excellent case it's never interested me, even though the .284 might even be more sensible than the 6.5-284. After all, I wanted the perfect cartridge.
I grumbled, wishing I could wring more out of the '08 case. What I really wanted, was literally scaled up 6mm BR to 7mm. Enter the WSSM's... By the time I had my reamer in production with Pac Tool and gauge, Winchester had come out with the .25 WSSM, making my necking up job that much easier. While the WSSM's flopped, the case was a godsend for me. Short, fat, high pressure. Everyone asks why on earth I went with 7mm over 6.5 and shakes their heads and sigh at what could have, in their minds, been better. Not so, there's good reasoning. The 7mm VLD's have higher BC's, and I wanted to take advantage of the high pressures, 63,000psi or more, with my AIM case and the 7mm has better basal hydraulics. Many are aware, but I'll review for those who are not; when dealing with hydraulics, which pushing a bullet down a bore is a function of, there are two ways to increase the force exerted on the piston (the bullet in our case). In a fictional scenario where we are doubling the force, we have two options counting out more involved approaches that would muddy this example; double the pressure, OR, double the area of the piston. This is why a .260 will push a 100gr bullet faster than a .243. 7mm, with that 63,000psi, struck me as the right compromise. "You'll never hit any meaningful velocities, you're asking it to compete with 7mm Mag on HALF the powder." Well, as I'll detail below, it doesn't do too badly at all and more than holds its own, and load development's still progressing.
The 7mm AIM, is almost literally a scaled up 6mm BR in proportions. I went with a 30º shoulder, and a long freebore for the Berger, then since I was already trying new things, I figured why not toy with the barrel too. I decided to go polygonally rifled, as the concept has always struck me as superior. Less bullet deformation, higher retained BC's as a result, longer accurate life, and higher velocities versus cut rifled. Now, it's not a revolution in bench rest, as shooters aren't dropping their single point cut rifled barrels by any means, but since I was already moving away from convention and wanted every drop of velocity I could squeeze, I went for it, and ordered a 7mm polygonally rifled pac-nor super match with a 1:8" twist in stainless.
Making cases was simple, one pass through my neck sizer, and a fireform with regular loads, then annealing and reloading. So far, in preliminary load development, velocities are on the mark, 2750fps with H4350 and Varget, and outstanding accuracy, with the cases handling the loads with ease.
The donor action is a trued M700 SPS .300 WSM, stocks tried were lone wolf palma, AICS stage II, and finally and settled on the HS precision pictured. The scope is a Leupold Mk4 M1 16x40mm, in Mk4 low rings, atop a 20MOA base.
Ballistics: (If the table gets scrambled, my apologies, fingers crossed)
Yards -Inches Vel Mach Energy Time
0 -1.5 2750.0 2.415 3022.1 0.000
100 -0.0 2634.7 2.313 2773.9 0.111
200 -3.5 2522.2 2.215 2542.2 0.228
300 -12.5 2412.5 2.118 2325.9 0.349
400 -27.4 2305.5 2.024 2124.2 0.477
500 -48.9 2201.2 1.933 1936.2 0.610
600 -77.6 2099.6 1.844 1761.5 0.749
700 -114.2 2000.7 1.757 1599.6 0.896
800 -159.4 1904.8 1.673 1449.9 1.049
900 -214.3 1812.0 1.591 1312.1 1.211
1000 -279.7 1722.5 1.512 1185.7 1.381
1100 -356.9 1636.7 1.437 1070.5 1.560
1200 -447.0 1554.8 1.365 966.0 1.748
1300 -551.6 1477.1 1.297 871.9 1.946
1400 -672.1 1404.2 1.233 787.9 2.154
1500 -810.2 1336.4 1.173 713.7 2.373
1600 -967.9 1274.2 1.119 648.8 2.604
1700 -1147.0 1218.2 1.070 593.0 2.845
1800 -1349.6 1168.6 1.026 545.7 3.097
1900 -1577.8 1125.5 0.988 506.2 3.359
2000 -1833.4 1088.2 0.956 473.2 3.631
Supersonic to 1800 yards on 44gr of Varget, 47gr of H4350. Calculation at 2500ft, 15% humidity, 75F.
Preliminary loading results, all in 27" barrel:
44.0 Varget 2750fps
47.0 H4350 2750fps (room for work up slightly)
40.0 H4895 2650fps
50.0 Retumbo 2460fps (case full)
Goal realized, I'm hitting 2750fps comfortably, and have a lot of powders to try yet. As well, on testing this rifle shot in the .2's and .3's at 100 while doing the chrono work with untested loads.
I will be extending the reach as the weater warms, out to a mile, hopefully. Verdict: 7mm AIM (my initials) flies, and very well, a worthwhile project.
Chrono results in:
40.0gr H4895 180gr Berger 2650fps pressure high, safe, but stiffer bolt lift
47.0gr H4350 180gr Berger 2750fps pressure warm but not really hot yet
44.0gr Varget 180gr Berger 2750fps pressure high
50.0gr Retumbo 180gr Berger 2460fps (just can't fit enough into the case, very low pressure)
This meets or exceeds 7mm Magnum on a LOT less powder, with correspondingly less blast and recoil. I credit the polygonal bore and the stout, fat little high pressure case (WSSM loads routinely run 62,000psi).
Barrel's actually only 27", I mislabelled it as 27 1/2" earlier.
All in all, an impressive showing. I want 2800fps though, anyone have any powder suggestions? I'm going to try a work up on 4350 a bit as there's room for a bit more there, but it'll be a bit warm for my liking. So if you have a powder in mind you think would work well here, please let me know.
As a side note, this thing will send 7mm AMAX's sailing, as they're only 162gr, I will try them next. So far the Berger's shooting insanely well though.
RJ
