Re: 260 rem, 7mm08 7mm08ai, which to choose
<div class="ubbcode-block"><div class="ubbcode-header">Originally Posted By: diggler44</div><div class="ubbcode-body">well i have typical rifle a.d.d. and am going to build a new rifle which will be a skunkworks 911 standalone action with cg jackson trigger in a manner t5a with chassis. i will be running aw mags so i will have alittle over 2.9" to play with. i was planning on the 260 but for some reason i always strive to be alittle different, plus my current match rifle is a 6x47 lapua which isn't that much different than the 260. i recently built a 284 and really like it, but dont think the casing will work well in aw mags so i am considering the 7mm-08 and the 7mm-08 ackley. i really like the 162 amaxs and would be building the barrel to shoot them.
so i am looking for opinions and experiences with the 7mm-08 and ackley version with the heavy bullets. i don't know what velocities to expect the the different calibers. please let me know barrel lengths and twists. i would think the 9 twist would be the key but not 100% with the relatively small 7mm case.
thanks for your input
scott </div></div>
Scott,
Lots of questions here, let me see if I can sift the chaff from the wheat.
First, either the 260 Rem (or 6.5 Creedmoor for that matter), 7mm-08, or 7mm-08AI are all excellent rounds that perform well out to 1000 yds and beyond, with good barrel life.
So which is the best? I don't know, but we can take a good stab at analyzing them and see how they compare.
First the 260 Rem w/140 gr Berger VLD. If you peruse the reloading forum you see loads that approach 2850 FPS, but most are in the 2800 FPS range, with 26" or shorter barrels. Let go with 2800 FPS, and plug that number into JBM using the Litz G7 BC.
1000 Yds -8.3 Mils -28.5 MOA Drift 1.9 Mils 6.5 MOA
Next the 7mm-08. Loads using the 162 A-max run around 2725-2750 FPS in barrel lengths less than 26". Let's use 2725 just to be conservative.
1000 Yds -8.9 Mils -30.7 MOA Drift 2.0 Mils 7.0 MOA
How about using a slightly better bullet, like the 168 Berger VLD at the same 2725 FPS?
1000 Yds -8.8 Mils -30.2 MOA Drift 2.0 Mils 6.7 MOA
How about the 7mm-08 AI? Well the 7mm-08 case is already fairly improved, blowing it out to a 7mm-08 AI only nets about 50-75 FPS max increase at equal pressures.
Taking the same Berger 168 VLD and increasing the velocity by 50 FPS, nets you the following:
1000 Yds -8.4 Mils -28.9 MOA Drift 1.9 Mils 6.5 MOA
There you have it. I'm not going to tell you which one to build, but they are all in the same ballpark performance wise.
As far as twist goes with a 7mm-08 regular or improved. Berger recommends 1-9" with bullet weights up to 180 grains. Bryan Litz opines that 1-8.7" is optimum for heavy 7mm bullets, and Sierra recommends 1-8.5" right on the box for their 175 Gr .284 SMK.
I'm currently building a 7mm-08 with a 1-8" twist Brux barrel, on the sole theory I would rather over-twist a little than under-twist a little.
IMHO, and YMMV,
Bob