Powder Burn Rate and Gas System Timing

[StoneHendge]

My 18" 243 LBC is a middie and suffers from gas timing issues. I've been using Benchmark and 95 grain SMKs. Even though Benchmark is relatively insensitive, I'm intrigued about trying Varget given that it's the least temp sensitive powder out there and I should probably be able to get within 50 fps of the 2740 I'm doing now. Will a slower powder make timing better or worse? I think this through in different ways and come up with a different answer each time (including "it doesn't matter"). Benchmark is a relatively fast powder so Varget would be slower.

[grayfox]

Wonder if someone out there with QL can run a couple of traces for you.
I know Klem on the 65 Grrrr forum has QL; however, yours being a 243lbc... a slightly different caliber. But, the concept should also be observable in a grendel, just have to choose 2 powders about the same delta speed as your two here.
Saying all of that, my guess is, and it's only a guess, that Varget will push the pressure peak out towards the muzzle a bit.
This also brings up the question, which way is better for the barrel and port (not sure that I know)? I guess you're looking for something to push the curve out towards the port so as to have more pressure there? and to cycle a bit harder as a result?

[BluntForceTrauma]

My 18" 243 LBC is a middie and suffers from gas timing issues.

What are your timing symptoms? Overgassed or undergassed?

[StoneHendge]

My 18" 243 LBC is a middie and suffers from gas timing issues.

What are your timing symptoms? Overgassed or undergassed?

Gas is hitting the bolt while the bullet is still in the barrel and i get ejector marks. They go away when I shut the gas off so it's trying to extract while the brass is still gripping the chamber. 

Grayfox, the objective really is just to be able to use the most temp insensitive powder possible (which is Varget) and I'm willing to sacrifice a bit of speed for it. I just don't want to make timing and hence stress on the extractor worse. You brought up something that I think answered the question but I'm going to hold off to see if anyone else has thoughts.

[BluntForceTrauma]

What happens when you adjust the gas lower? Sounds like it's overgassed with the gas port too big for the mid-length position.

[StoneHendge]

What happens when you adjust the gas lower? Sounds like it's overgassed with the gas port too big for the mid-length position.

It's set to where it cycles reliably. 2 clicks down and it won't cycle reliably.

[Dino11]

How bad are the swipe marks?

Light swipes are not something to worry about. But if they are deep and very pronounced that could be a problem.

Maybe try changing out the ejector spring, yours may have a little to much tension on the ejector. And you might try removing the "O" ring if your extractor has one.

[StoneHendge]

How bad are the swipe marks?

Light swipes are not something to worry about. But if they are deep and very pronounced that could be a problem.

Maybe try changing out the ejector spring, yours may have a little to much tension on the ejector. And you might try removing the "O" ring if your extractor has one.

They aren't bad - no worse than what you might see with M193 in a 16" .556 with a carbine gas system.

I've been thinking about ditching the o ring. But first, I remembered I have a Taccom 10% reduced power spring in my parts bin. Went to swap it out and it looks like I've been running it with a 4.3 oz buffer which was from the rifles days with Saturn Liberty 6.5G barrel with an oversized gas port. Dropped a standard buffer in there with the reduced power spring. Even if the marks don't go way when I gas it down, at least there will be less stress on the extractor while it yanks and waits for the brass to relax.

[StoneHendge]

Well I think I got this answered. Being fed up with the temp sensitivity of CFE 223 in my 6.5 Grendel load, I loaded up a ladder with 8208 XBR and kept everything else the same (Lapua brass,123 Scenar, GM205M). I had been getting 2600 fps with 30.4 grains of CFE (31.2 book max) at 95 degrees ambient (ammo stored at 80). Was 95 this morning, ammo had been stored at 80, and I started getting ejector marks at 27.2 grains (28.0 book max). XBR is quite a bit faster than CFE so I'm gonna guess slower powder helps timing. The 6.5G barrel is 22" JP with RLGS, JP LMOS carrier and tuned silent capture spring.

[StoneHendge]

Well, I don't know if the questions been answered, but the result appears good. I got the 95 gr TMKs going 2656 sd 2 es 7 (2660, 2653, 2658, 2558, 2655) with 28.2 grains of Varget (Hornady ARC max of 28.1 for 2600). Case heads were fine at 28.0 and I got very light ejector marks at 28.2. I have the lighter (standard) buffer in and the 10% reduced power spring, but haven't reduced the gas. I'm guessing when I reduce it, the marks will go away. 5 shot group was 1" tall with Magneto Speed strapped on and my Bipod had become loose (lever all the way to the rear). I'm guessing the latter strung it vertically - the horizontal dispersion was 0.3"). Time for more testing but I'll certainly trade 90fps for not breaking an extractor at the wrong time along with Varget being the most temp insensitive powder out there.

I need to do more testing but I also got 70 grain NBTVs up over 3100 with 30.1 gr of XBR. Four shots into a clover leaf you could cover with a dime, but a Stone Hendge blew it out to 3/4". No velocity data due to a brain fart, but 29.8 gr had given me 3094 sd  es 24. Above 3100, that's hitting noslers  1400 fps minimum threshold for expansion at 650 yards.

[Dino11]

I had a similar problem as you. My rifle has a rifle buffer and spring in it. I was getting light swipes and it would not lock back on last round fired. I had the GB opened all the way to. I removed 4 of the steel weights in the buffer taking from 5.2 oz down to 3.6 oz using aluminum stock I made my own weights and put in a reduced power buffer spring. I was able to close the gas block past half way and now it shoots like a pussy cat. Hardly no recoil at all, and I talking like your shooting a .22.

Being able to turn the gas way down was the ticket, and this barrel is rifle gas +2, the gas port is almost at the end of the barrel, but it is a whopping .101 in size. So it was violently unlocking but did not have enough omph to move the bolt back far enough to lock back. No more swipe marks.