Looking for thoughts on hot loads in an AR10

[dcmdon]

Hi all.
I'm looking for some thoughts on using hot .308 loads in my AR10. Here's the details.

The ammo is 175 gr Sierra Match King handloads that I developed to use in my bolt gun when I was taking the advanced precision scoped rifle class at Sig.
It chrono'd at 2720 FPS out of a 22 in barrel on my bolt gun.
It recoils noticeably more than 168 or 175 gr Federal Gold Medal Match

The gun is a rifle I built using a Mega upper and lower as well as a Lilja barrel with a bolt lapped to the barrel extension.
Its also got an adjustable gas block.

When I started shooting these rounds, I turned the gas way down and fired the first shot. There were no pressure signs on the case or primer and the case did not eject from the rifle.

I gradually turned up the gas over the next 4 rounds until the bolt just locked back on an empty mag.
With this level of gas, there were still no pressure signs on the case and also no signs of extraction happening when there was still pressure in the barrel. (deformed rim, gouges in the case, etc).

So by all measures the gun is functioning properly with the ammo.

My question is more along the lines of the strength of the bolt compared to a bolt gun bolt.
Is there a harm in this?

Why? You may ask?
I'd like to try my hand at shooting this AR out at extended distances. It is well sub MOA (average 5 shot groups, not a cherry picked 3 shot group) at 200 yards and this ammo has proven to have a very small standard deviation of 3 FPS.

Like I said, I developed it for a specific bolt gun, but it has proven to be very accurate in every gun I've shot it through. Including my A-Bolt Micro, which weighs 6.2 lbs with a scope. (ouch!!!)

Any personal experience on the durability of AR10 bolts would be welcomed.

Thanks,

Don

 

[Howland]

Tagged for interest as I had a plan with the same objective. I now realize that an adjustable gas block will not be optional. My standard configuration is way overgassed for 178s with common .308 powders.

 

[dcmdon]

Hmm. Bump.

 

[Bodhisattva]

I’ll give you a hot load, oh wait, shit wrong message board

 

[xtry51]

So the gas system does not effect the stress seen by the bolt. This is because the maximum chamber pressure is seen before the bullet ever reaches the gas port due to volume behind the bullet rising as it travels down the barrel. This volumetric expansion rate exceeds the burn rate of the powder.

Next point, the primer will fail before the case in *most* cases. This is dependent on number of reloads, but if you're getting full case separations in chamber, perhaps we need to discuss your brass inspection process

I personally load up until I see cratering of the primer cups around the firing pin. This is an indication you are getting close. On 308 you're likely about 0.5-1.0gr away from a pierced primer when you can catch first the crater with a fingernail. This will vary with powder/bullet/primer cup combos.

As to the bolt strength, it will exceed the brass/primer strenght and if you're ladder loading you will see all sorts of pressure signs before you could kill a bolt. Primer flattening Primer crater. Base thinning. Neck cracks. Pierced primers.

If you are carefully ladder load to primer flattening, you're well below anything remotely hard on a bolt.

 

[Yazz]

What is the twist on the barrel?
And what kind of buffer do you have on the AR-10?

 

[xtry51]

I just looked in my book. 180gr 44.0gr RE15 was my go to on my Savage for most accurate at 300yds. No pressure signs. Max listed is 45.0gr. Not sure if that helps.

 

[xtry51]

Here is a cool study on pressure by barrel length and along length for 5.56. Max pressure when projectile is 3 inches into bore.

https://ndiastorage.blob.core.usgovcloudapi.net › ...PDF
Web results
EFFECTS OF BARREL LENGTH ON BORE PRESSURE, PROJECTILE VELOCITY and ...

 

[92G]

It’s negligible benefit of pushing MV to another 50 or 100 ft/s. How much more distance is achieved to trans-sonic? Not much, maybe an extra 25 meters?

my personal preference would be to not shoot any 308 that recoils noticeably more than GMM loading w the same projectile. As momentum is conserved, mass being equal, velocity must be significantly more. the benefit to risk isn’t there. I would be more concerned with a case or primer rupture especially being on margin of pressure limits. But don’t let me talk you out of it.

 

[dcmdon]

So the gas system does not effect the stress seen by the bolt. This is because the maximum chamber pressure is seen before the bullet ever reaches the gas port due to volume behind the bullet rising as it travels down the barrel. This volumetric expansion rate exceeds the burn rate of the powder.

Next point, the primer will fail before the case in *most* cases. This is dependent on number of reloads, but if you're getting full case separations in chamber, perhaps we need to discuss your brass inspection process

I personally load up until I see cratering of the primer cups around the firing pin. This is an indication you are getting close. On 308 you're likely about 0.5-1.0gr away from a pierced primer when you can catch first the crater with a fingernail. This will vary with powder/bullet/primer cup combos.

As to the bolt strength, it will exceed the brass/primer strenght and if you're ladder loading you will see all sorts of pressure signs before you could kill a bolt. Primer flattening Primer crater. Base thinning. Neck cracks. Pierced primers.

If you are carefully ladder load to primer flattening, you're well below anything remotely hard on a bolt.

I didn't mean to imply that the gas system affected stress on the bolt. I just wanted to emphasize that the rifle is properly timed and functioning flawlessly.

Thanks for the info on primer cratering. The primers look fine. no cratering, no flow.
So I guess all is good.

What is the twist on the barrel?
And what kind of buffer do you have on the AR-10?

Barrel twist is 1:10. Buffer is irrelevant since I'm tuning the action with an adjustable gas block.

 

[dcmdon]

It’s negligible benefit of pushing MV to another 50 or 100 ft/s. How much more distance is achieved to trans-sonic? Not much, maybe an extra 25 meters?

my personal preference would be to not shoot any 308 that recoils noticeably more than GMM loading w the same projectile. As momentum is conserved, mass being equal, velocity must be significantly more. the benefit to risk isn’t there. I would be more concerned with a case or primer rupture especially being on margin of pressure limits. But don’t let me talk you out of it.

My motivation for using these rounds is pretty simple. I already have them and they are very accurate.

In preparation for the Sig precision scoped rifle class I went all OCD and once I found a load that was accurate in my bolt gun, I made up 600 of them over the course of many many hours.

I REALLY went nuts. I bought 1000 pieces of brass. Full length resized all of them, reamed the flash hole to a consistent size, then trimmed all to length. Then I weighed all of them and selected the 600 that all had their weight within the tightest group. I used some of the other 400 cases when we were shooting out to 300 yards, where slightly greater SD in muzzle velocity would make no difference.

Since I had defined the exterior dimensions with the die and trimming, weight difference was indicative on variations of interior volume. By making the interior volume more consistent, I created an environment for more consistent chamber pressures. This gives more consistent muzzle velocities. My 3 fps standard deviation in MV is a big deal at long distances.

For example, a 30 fps (typical for commercial ammunition) variance in 2 rounds shot at 100 yards will be .1 inch. The difference in time of flight simply isnt' long enough for the MV to make much of a distance.
But at 1000 yards, a 30 fps variance will result in a 40 inch difference in elevation. More than enough to make the difference between a hit and a miss.

At the Sig class I shot only about 250 rounds of this batch. So I have roughly 350 rounds of extraordinarily accurate ammo that I'd like to be able to use in my AR10.

Its as simple as that. Its not about being able to go another 50 yards before it goes transonic. At standard temp and pressue (59F and 29.92) its above 1100 FPS until 1160 yards. More than good enough.

 

[dcmdon]

At the class after we chrono'd the ammo, the instructor Dave Hinkel told me something to the effect of "You are a decent shooter. But you are an AWESOME reloader". Ha.

 

[xtry51]

I took a scoped carbine class at Ridgeline. My 77gr .223 reloads chronod at 12fps SD. Instructor basically told me in so many words "it was OK but I wouldn't call it acceptable" rofl. Crushed my soul but didn't stop me from hitting human targets out to 600.

I don't have the patience to weigh out the brass.

 

[PatMcD]

Keep adding powder until you pierce a primer then back it off half a grain. That's a good, hot load.

 

[xtry51]

Keep adding powder until you pierce a primer then back it off half a grain. That's a good, hot load.

Goes to range to load test. One box of ammo, one box of new firing pins.

 

[Broc]

I have nothing to contribute, but I wanted to comment on this:

this ammo has proven to have a very small standard deviation of 3 FPS.

You must be an amazing reloader, or the weather was just right, or the planets aligned. 3FPS is NOTHING.

 

[dcmdon]

I have nothing to contribute, but I wanted to comment on this:
You must be an amazing reloader, or the weather was just right, or the planets aligned. 3FPS is NOTHING.

I just found the entries in my data book that we kept during the class.

Like I said, the instructor was impressed. He told me if he ever has to go back to the middle east, he wants me to make ammo for him. Ha.
Dave Hinkel was an excellent instructor, if you ever have the chance to take a class from him you should.

One thing I also did a couple of weeks before the class was to do a one day Appleseed to tune up my techniques. Appleseed is the best bang for the buck on the planet.
But even then my marksmanship was nowhere near the top of the class at Sig. At least I couldnt' blame it on the ammo. Ha.

 

[xtry51]

3fps is probably the SD of the tool you're using to measure. Lol.

 

[308rifleman]

I would stick with M80 ball or M118LR ammunition for a semi-auto .308. Why take a chance and possibly ruin a nice rifle? If you want to experiment with hot handloads, you might be better off purchasing an inexpensive bolt-action or single-shot rifle.

 

[DarthRevan]

 

[PatMcD]

I would stick with M80 ball or M118LR ammunition for a semi-auto .308. Why take a chance and possibly ruin a nice rifle? If you want to experiment with hot handloads, you might be better off purchasing an inexpensive bolt-action or single-shot rifle.

Anything that would ruin/detonate a AR10 would ruin/detonate a bolt rifle too. They're pretty foolproof.
A M14- type is a different story.

 

[PatMcD]

Keep adding powder until you pierce a primer then back it off half a grain. That's a good, hot load.

Everybody is laughing at my post, but that's basically how hot loads were developed for the AR15: experimentation. Obviously you don't go overboard with powder charges, but you can easily go a grain or two up from the published loads. Want more speed? Add more powder. Pop a primer? You added too much; back it down a little.
AR bolts are a consumable item, just like barrels.

 

[Mesatchornug]

Everybody is laughing at my post, but that's basically how hot loads were developed for the AR15: experimentation. Obviously you don't go overboard with powder charges, but you can easily go a grain or two up from the published loads. Want more speed? Add more powder. Pop a primer? You added too much; back it down a little.
AR bolts are a consumable item, just like barrels.

I should be clear, I thought I was laughing with your post. I read it as equal measures of literal and snark. If I misunderstood, you have my apologies.

 

[dcmdon]

Everybody is laughing at my post, but that's basically how hot loads were developed for the AR15: experimentation. Obviously you don't go overboard with powder charges, but you can easily go a grain or two up from the published loads. Want more speed? Add more powder. Pop a primer? You added too much; back it down a little.
AR bolts are a consumable item, just like barrels.

The "problem" I faced when developing this load for my bolt gun was the hotter I made it, the more accurate I got. So the motivation was to keep going until I just didn't want to make it any hotter. Often with load development, you reach a point where as you go up, precision starts to degrade. So you stop. That didn't happen in this case.

When I was developing loads for my service rifle AR15 I reached a point of maximum precision pretty close to my starting powder charge. So they're pretty weak. But I'm usually competing at 200 yards so mv is pretty much irrelevant.

 

[Davidk]

Just because the load is accurate in your bolt gun does not mean it will be accurate in another rifle

 

[Queen Bee]

I have also built a mega arms set but in 6.5 creedmoor. Working on getting an adjustable GB on it because of the overgassed setup without one and having primers blow, and cratering.

 

[wiryone1]

At the risk of wondering off topic slightly, would it not be better to measure the volume case to case rather then the case weight? I mean the weight of the volume of water in a case with a plugged primer flash hole comparing that weight to sort cases. It would seem you could have a two cases theoretically that had identical weights but slightly different volumes. Or maybe two cases with the same volume but different weights. Isn’t weighing empty cases just telling you what the consistency is of the amount of brass that end up in the case. What if one case had a thicker wall while another had more material in the head?

I am just thinking. Maybe doing water volume is way too much work. In no way is a SD of 3fps bad. Whatever you are doing continue on as it obviously works.

 

[dcmdon]

Just because the load is accurate in your bolt gun does not mean it will be accurate in another rifle

You clearly haven't read anything here. I specifically said that it had proven to be exceptionally accurate in my AR also.

If it wasn't accurate, there wouldn't be any motivation to use it.

 

[dcmdon]

I have also built a mega arms set but in 6.5 creedmoor. Working on getting an adjustable GB on it because of the overgassed setup without one and having primers blow, and cratering.

I've used the aluminum JP blocks on 2 rifles, including this one. They are very nicely made and very light. Obviously not for hard use because they are aluminum.

 

[dcmdon]

At the risk of wondering off topic slightly, would it not be better to measure the volume case to case rather then the case weight? I mean the weight of the volume of water in a case with a plugged primer flash hole comparing that weight to sort cases. It would seem you could have a two cases theoretically that had identical weights but slightly different volumes. Or maybe two cases with the same volume but different weights. Isn’t weighing empty cases just telling you what the consistency is of the amount of brass that end up in the case. What if one case had a thicker wall while another had more material in the head?

I am just thinking. Maybe doing water volume is way too much work. In no way is a SD of 3fps bad. Whatever you are doing continue on as it obviously works.

That's a great question. But you are forgetting about the case prep done before they are weighed.

The cases are full length resized and then trimmed to length. Which means their exterior dimensions are the same. So if their exterior dimensions are the same, then interior volume is a direct reflection of weight. Heavier cases will have greater wall thickness and lower volume.

If I hadn't first full length resized them and trimmed to length, your point would be valid. But by doing this, we are making the exterior dimension a constant.