Pressure signs, hot barrel.

[58blackflag]

I am trying to work up a new load for my .260 bolt gun. From a previous rifle I was using 139 scenars, 41.7 grains of RL-17, CCI BR2 primers, all in Lapua brass. I have fired quite a few of the leftover old loads thru the new gun but every now and then I get a sticky bolt lift or ejector mark on the case.

When trying to find something a little milder, yesterday I used 41.5 grains of powder instead of 41.7. I shot ten rounds of the old load without any pressure signs then the first 5 rounds of the new milder load showed ejector marks and the bolt lift on the first two shots were a little sticky.

Could this be strictly from the gun being hot? I am using a suppressor so it def heats up more quickly.

The old load is also more consistent with a SD below 10 and a smaller ES. Groups are all pretty consistent with my shooting being the limiting factor.

As a barrel gets hot can you expect more pressure signs and a loss of consistency with chrono data?

 

[Joey Dean]

Funny you should ask.

I got a new Magneto Speed V2 two weeks back and had to go and play. Chrono'd some loads and got to the end of the day with 20 rounds left for my AR that were not going to be keepers. These were all hand weighed in full prepped brass. Shooting slow fire groups earlier in the day I found they had a sd of 12 for 3 x five round groups. I loaded these into one magazine and started shooting them as fast as I could come back onto target out of my RRA 20" AR.

The last three rounds were running 75 fps faster on the average than the first three. This was a very mild load running 2980 average during the slow fire done earlier.

 

[SmokyJoe]

Some research I found shows that much of the pressure/velocity increase comes from the primers getting heated. Their tests showed that in a hot rifle the primer gets heated quickly, much faster than the powder.

 

[Dan Newberry]

Even the vaunted Varget will run the numbers up when it's 90 degrees out and you're shooting a longer string of fire. With new brass, you may not see the problem, but on the second or third firing, you'll find loose primer pockets... I experienced this myself with some hot loaded .308 that I shot, then went to reload. Second firing, but primer pockets were already loosening...

As you get closer to the tipping point (at which the primers flatten, bolt lift gets sticky, or worse, you puke a primer), the heat will affect you more.

This is yet another good reason not to "drive it like you stole it"... find a good accuracy node on down from where you are now, and things won't get quirky on you when it's hot outside, and you have to shoot longer strings of fire.

Brass is hard to find these days, and not generally cheap when you do find it.

All this said... the RL17 may be the problem. It's getting a terrible rep for misbehaving in temp swings.

Varget at 38.0 grains for your 260 shooting the 139's (or 140's) will serve you well... but of course Varget is pretty scarce these days...

Dan

 

[58blackflag]

Thanks for the recommendations Dan! I actually have plenty of Varget on hand, it's what I use primarily for .308.

What kind of velocity can I expect out of 38 grains and a 140ish grain bullet?

 

[Dan Newberry]

Several of the guys I shoot with are using the same load, and getting 2700 fps from 24 inch barrels, and a bit more from 26's... it's extremely accurate.

 

[58blackflag]

Gotcha, I'm getting 2825 out of the current load with a 24" barrel. I'm trying to get a load together for a trip out west to participate in a ThunderBeast training class. I'm expecting to be taking shots out to 1000 yards and beyond so the extra MV is nice to have. In the last class i went to we shot all the way out to a mile.

Good thing about the location is temps should be milder than here so hopefully my RL-17 won't be too temperamental. It was in the mid 80s when I was shooting the other day and had pressure signs.