260 AI speed question

[huckjare]

Well that 260AI I had before has since hit the road. I just finished a build with a new one though. Defiance action, proof research barrel, bansner stock, etc. I've decided to give the Barnes LRX bullets a try. I've got some fire formed brass loaded with RL17 but haven't been out to see how it shoots. With that small bullet at 127g I should be well over the 3000 fps mark. Wight the BC and SD of that bullet I should have enough energy to take elk out to 900 yards without an issue.

 

[Trickymissfit]

Hornaday list several loads with a 140 grain bullet. All maxed out at 2700 fps using a 24" barrel in a 700 Remington. Hornaday does not list pressure specs, and this is all important. Accurate does list chamber pressure specs on all their loads, and the max SAMMI pressure is 60K psi. AA also used a 24" barrel in an H S Prec. rifle (similar to a 700 Rem.) Here's what they got with a 140 grain bullet:

SWF 140 SP, 41.5 grains of AA 4350 @ 2670 fps with 58K psi

Speer 140 SP, 41.0 grains of AA4350 @ 2669 fps with 59.6 K psi

Nosler 140 Part, 42.5 grains of AA 4350 @ 2733 fps with 59.4K psi

SRA 140 SBT, 43.5 grains of AA 4350 @ 2740 fps with 59.1K psi

HDY 140 SP, 43.5 grains of AA 4350 @ 2740 fps with 60K psi

SRA 140 HPBT, 43.0 grains of AA 4350 @ 2767 fps with 60K psi

SRA 142 HPBT, 43.0 grains of AA4350 @ 2744 fps with 60K psi

Now you may notice I only used AA4350 data. AA3100 often gave higher velocities, but often less than 15fps. Plus 3100 is now extinct. 2700 shot pretty well also, but AA4350 showed at the top almost everytime. I was surprised that they showed no loads using 2495 and maybe even Magpro. 2495 was developed for the .308 case, and works very well. I'd be interested is what 2495 and Magpro would do with the 140 grain class of bullets.

Looking at the improved designs, I can see maybe a grain and a half gain in power capacity. Without much of a serious gain, and less than 100fps the advantage just doesn't show. Plus I suspect some of those wild velocities are approaching 70K psi, and looking to send you to the trauma ward.
gary

 

[stomp442]

I know this is an older thread but this is my input. What cases are you using? If necking down brass you really need to turn the necks for best performance. I have a savage chambered in this cartridge with a 26" criterion barrel. The sweet spot for my barrel is 2940 fps and I can achieve that quite easily in fact I had velocities over 3000 fps during load development but it was hard on primer pockets so I backed it off. I am using brass that I made from 30-06 lake city match cases that are trimmed to fit my chamber exactly. By using 30-06 cases I am able to have a slightly longer neck than people that simply fire form 260 or 243 cases. The match cases are also thicker and provide higher velocity than standard brass while using less powder. My load for 140 bergers in my AI is only 44 grains of H4831 that's giving me 2940 fps and less than 2" groups at 600 yards. I realize some barrels are faster than others and I'm sure I have a faster one but using match cases ups my velocity by 60 fps vs standard Remington brass. To achieve the same velocity using Remington cases I have to use a grain and a half more powder. My point to all this being that maybe using match cases could get you to the velocity you desire. its a lot of work to make custom cases but I feel its worth it in the long run as it greatly decreases throat erosion and extends barrel life. Shoot me a pm if you would like step by step instructions on making cases.

One other thought. Have you ran your rifle through multiple chronys to verify speed?