XBR8208 Pressure spike too fast for setup?

[CheapBasterd]

Is XBR8208's pressure spike too fast?
I could not get it to cycle reliably in a 20" 1/7 rifle (gas & buffer) length system.

2560 FPS / 75gr BTHP and it was basically a single shot AR

With Varget at the same FPS Avg (2556 FPS) it would cycle perfectly and lock back on the mag.

I had another set of loads with XBR running 2800 FPS with 68gr HPBT. These would cycle but it was still short stroking on some rounds and would not lock the bolt back.

All these loads work just fine in my midlength setups.

 

[supercorndogs]

I have always had good like with mine running on a 77smk or nosler cc and 23.2g of 8208.

 

[375fan]

What's your powder charge with the 75gr?

 

[CheapBasterd]

What's your powder charge with the 75gr?

21.5gr XBR

23.5gr Varget

2.250” COL
CCI 450

 

[375fan]

I have 16" w/ 7tw , my thought is your powder charge with 8208 is a tad light and that's why its not cycling properly.

 

[neeltburn]

I have seen 8208 spike pressure when nearing max loads with bolt guns. Go up slow and find the sweet spot. Look at you cases and primers for signs up pressure before you near max load. What case are you using? Case volume is a factor.

 

[CheapBasterd]

I have seen 8208 spike pressure when nearing max loads with bolt guns. Go up slow and find the sweet spot. Look at you cases and primers for signs up pressure before you near max load. What case are you using? Case volume is a factor.

LC brass

No visible pressure signs on any of these loads out of this rifle. I’ll try dialing it up a bit.

 

[neeltburn]

LC brass

No visible pressure signs on any of these loads out of this rifle. I’ll try dialing it up a bit.

Go up slow. LC brass is thicker than 223 Rem brass so it will have a lower case capacity. ( Currently seeing this first hand in a 308 gasser) This might yield a bit slower velocities but should still allow you get to a working load. Watch your cases and primers as you load up.

 

[supercorndogs]

I shoot 23.2 in LC brass.

 

[DHDeal]

I shoot 23-23.5 of XBR with many different 70-77 grain bullets. The shortest gas length I have is called a Carbine +1 on a Craddock 10.5"? Bartlein and all the way up to a rifle length on a Proof and 23-23.5 cycles every one of them without any hiccups even with a heavier buffer.

Put some more XBR in that case kind Sir and enjoy. I wouldn't worry about whether a LC case has less volume or not with a 23-23.5 grain load of XBR.

 

[CheapBasterd]

Well shit. Went out today to test up to 23.2gr of XBR still not enough. 2723 FPS, No pressure signs, still full radius on the primers. Should have went up to 23.5+

Curious noob question. Why would my other loads with lower or even same velocities cycle okay? Is this the high pressure low velocity readings about XBR8208?
Varget @ 2556 FPS - Cycles fine.
TAC @ 2716 FPS - Cycles fine.

FWIW every sample was around MOA or SUB even when throwing the jammed up dented cases back in. Seriously impressed with the WOA service barrel.

 

[neeltburn]

Do you have an adjustable gas block by chance?

 

[CheapBasterd]

Do you have an adjustable gas block by chance?

I do, a superlative bleed off. It’s set wide open 18 clicks (4.5 turns).
The thought of my gasblock being misaligned did cross my mind but other loads are cycling fine.

 

[375fan]

I do, a superlative bleed off. It’s set wide open 18 clicks (4.5 turns).
The thought of my gasblock being misaligned did cross my mind but other loads are cycling fine.

When you say other loads are cycling fine, are you running other reloads (different bullet/powder combos) or factory ammo?

 

[CheapBasterd]

When you say other loads are cycling fine, are you running other reloads (different bullet/powder combos) or factory ammo?

My other reloads that I have developed for my other AR's.
All 75gr Hornady at 2.250"
Varget @ 2556 FPS - Cycles fine.
TAC @ 2716 FPS - Cycles fine.

The only problem im having is with this new rifle and the XBR loads. So its not out of the question yet if something is up with this rifle.

 

[Doom]

Your velocity is telling you your charge weight is low and the gas system is also telling you the same thing. Most people find the 77 gr Sierra'a, Noslerswhich are very similar bullets run around 23.2 which is Hodgdon's Max.

Go up slow. LC brass is thicker than 223 Rem brass so it will have a lower case capacity. ( Currently seeing this first hand in a 308 gasser) This might yield a bit slower velocities but should still allow you get to a working load. Watch your cases and primers as you load up.

Nope. Not true in 223/556.

 

[DHDeal]

Nope. Not true in 223/556.

That particular wive's tale likes to live on. I decided not to type it but glad someone did.

I don't think anything is wrong with your rifle OP as it functions with other loads. I have and use the SA and SLR gas blocks (more SA than SLR though). I don't use the SA in the bleed off settings as a whole. You are at a point of tuning and possibly needing a touch more XBR8208.

Has accuracy improved with more 8208? Are groups more vertical than horizontal or are they round? In my case, I have never had any concern that 8208 would not cycle completely and the lightest bullets I reload are 65 grain SGK's (I use another powder there though). When I use 8208 I load either the 75 grain HPBT (what you are using) or the 77 SMK. I tend to load 23.5 grains but wouldn't hesitate to go to 23.7 grains if it was more accurate for my rifles. I do use the same loads in all of my AR's and they function fine in every one. I do use suppressors on every one though but that particular load functions with and without a can.

That last paragraph is fine and well (for me) but in your rifle it doesn't work. When I put my rifles together I will use my bore scope to look at the gas port/gas block from the inside and I also run the SA gas block in the restrictive mode. I am a little surprised yours doesn't cycle 23.2 grains but will cycle 23.5 grains of Varget. Your 23.5 grain load of Varget is a grain less than I use but it cycles for you. I would go restrictive mode and load 1 round in a magazine at a time while adding a "click" at a time until I get lock back. I would start at 5 and move out/open. When I get lock back I add 2 more "clicks" as a general rule for me. I hope this works for you!

 

[Doom]

I'll add a little additional observation on 8208 as I've been a user since shortly after it was introduced. The guys who originally tested it loved it. I believe it was Lou Murdica and Jim Carmichel tested and won matches with it. After it was released with the original load data people seemed to have issues with it until it was at the max published loads. It appeared to some that the released lots were slower than the original test lots and that the load data was low. Whether that is the case or not I do not know.

In regards to the Varget load and the 8208 load at similar velocities, Varget is a slower burning powder and therefor at the same muzzle velocity should have a lower chamber maximum pressure and a higher gas port pressure than the faster burning 8208.

I'm guessing you also have a rifle length gas system so loads that work but are borderline in shorter barreled mid and carbine gas systems may not function yours.

 

[NHPiper]

Have had good luck with 23.2-23.5gr of 8208 and 77 SMK. The 23.5 started to show pressure signs in both my 16” and 18” barrels. It definitely has a more aggressive pressure spike than Varget and doesn’t deal with being compressed as well.

 

[DHDeal]

I'll add a little additional observation on 8208 as I've been a user since shortly after it was introduced. The guys who originally tested it loved it. I believe it was Lou Murdica and Jim Carmichel tested and won matches with it. After it was released with the original load data people seemed to have issues with it until it was at the max published loads. It appeared to some that the released lots were slower than the original test lots and that the load data was low. Whether that is the case or not I do not know.

In regards to the Varget load and the 8208 load at similar velocities, Varget is a slower burning powder and therefor at the same muzzle velocity should have a lower chamber maximum pressure and a higher gas port pressure than the faster burning 8208.

I'm guessing you also have a rifle length gas system so loads that work but are borderline in shorter barreled mid and carbine gas systems may not function yours.

I remember reading an article with that information. Probably written by Carmichel a long minute back.

I have noticed it has, for lack of a better description, a narrow window. I have never hot rodded it to "see what it could do" like I have some other powders probably because of that article. I will say when I use it for specific bullet weights and specific cartridges, it is a great powder with no surprises. For my purposes, it won't be the fastest but very likely the most accurate (under 120 grain bullets in 6.5 Grendel is a good example).

I think your second paragraph is correct. It's kind of what I was trying to type but didn't explain to well when I was suggesting going to restrictive mode with his gas block (not letting all that gas be wasted). Nice explanation!

 

[CheapBasterd]

The groups were all over the place but did shrink in ciruclar pattern as the charge weights went up from the original 21.5 up to the 23.2. I'm finding people working up to even 24gr (small subset of posters across forums). I will keep moving up the weight until I can get XBR to cycle in this particular rifle. Even at 23.2, no swipes, nice round primer edges so there is more room for this load. Yes, its rifle length.

I have another 20" rifle length gas system (FN Barrel/A2 FSB) and it cycles anything including that puff of 21.5 XBR. But that rifle is well over 20 years old, maybe larger gas port than the WOA.

After reading a couple posts here and there including here, im not too far off my though that it has a high pressure spike at the chamber and less gas at the port (more obvious in a rifle length gas system). This is definitely a case of reloading 101 but this powder seems to have unique characteristics. Even smells different, in a weird good kinda way...

oh yeah, i did check my gas block alignment with a borescope. It was fine. Not bad with eyeball install.

@DHDeal Was a while ago when I loaded those Varget rounds. If I recall, I was slightly crushing kernels at 23.5gr + 75 pill.

 

[DHDeal]

The groups were all over the place but did shrink in ciruclar pattern as the charge weights went up from the original 21.5 up to the 23.2. I'm finding people working up to even 24gr (small subset of posters across forums). I will keep moving up the weight until I can get XBR to cycle in this particular rifle. Even at 23.2, no swipes, nice round primer edges so there is more room for this load. Yes, its rifle length.

I have another 20" rifle length gas system (FN Barrel/A2 FSB) and it cycles anything including that puff of 21.5 XBR. But that rifle is well over 20 years old, maybe larger gas port than the WOA.

After reading a couple posts here and there including here, im not too far off my though that it has a high pressure spike at the chamber and less gas at the port (more obvious in a rifle length gas system). This is definitely a case of reloading 101 but this powder seems to have unique characteristics. Even smells different, in a weird good kinda way...

oh yeah, i did check my gas block alignment with a borescope. It was fine. Not bad with eyeball install.

@DHDeal Was a while ago when I loaded those Varget rounds. If I recall, I was slightly crushing kernels at 23.5gr + 75 pill.

When I stuff 24 grains of Varget under a 75 grain (or 77 grain) bullet, I ease down on the Co-Ax's handle. Gentle cracking is a way to describe it I suppose...

A little more 8208 then. For the heck of it try the restrictive mode. I'd love to be right here and you won't mind if I was.

 

[CheapBasterd]

When I stuff 24 grains of Varget under a 75 grain (or 77 grain) bullet, I ease down on the Co-Ax's handle. Gentle cracking is a way to describe it I suppose...

A little more 8208 then. For the heck of it try the restrictive mode. I'd love to be right here and you won't mind if I was.

Thats the plan. 23.5 , 23.8, 24.0 and yeah for the heck of it try some of the block settings. Unlimited use of an indoor 100 yard range, helps alot with load development and fuckery.

 

[NHPiper]

Be very careful at 23.8 and up. You may start getting into blowing stuff up territory. I'd expect to see primers start popping out or piercing at 23.7 in LC brass with 77s.

 

[DHDeal]

I personally would stop at 23.7 and that would be the 5 I loaded "just in case everything looks great and I just had to know" kinda thing. You can pull 5 easily enough. 23.5 will probably give you all the velocity you are looking for with this load IMO.

If you want screaming velocity (which you haven't mentioned) then a ball powder such as 2520 is one candidate.

 

[CheapBasterd]

Thank you guys for the caution. I hope to keep it safe as can be.