Need help with 450 Bushmaster loads.

[Wildcatter]

Then again you may have already answered that question, I think Pete said something about a thicker web, which would up pressures. I guess they must have lowered the powder load some? Just going from memory and I probably don't have it right yet.

[ar15barrels]

Interesting. I was taking to Pete Pi about a year ago and Starline was having some kind of a problem with the SOCOM Brass or was it the 50B brass or both, I forget. Anyways, something about splitting, supposedly they have corrected the problem. Do you know anything about that?

There was a bad batch of starline 458 a year or two ago.
It was a dimensional problem.

[Slash2]

Range update

I had a chance to test some loads today. All were loaded with Hornady XTP/MAG 240 HP.

Mostly I shot 36.5 grs of H110 with an AOL of 2.1 and 2.15". The 2.1" cronied at around 2150fps with the 2.15AOL running 50fps less. Both grouped around 1.5 to 2" @ 100yds.

Next I wanted to try some heavier loads. I started at 38.5 grs H110 and stepped up .5 grs a round till I was at 48 grs. All at 2.15 AOL

The fps stared at 2150. I didn't see much flattening of the primers until around 43.5 grs. the velocity was 2475 fps at that point. I fired 3 more and then decided to quit at 45 grs and 2550 fps. The primer was getting pretty flat but there was no sign of a crater.

The recoil at this point was substantial but the bullets were making a nice straight line, one above the next, over the x.

What's your opinion of the picture, is this a flattened primer, a slightly flattened primer or a dangerously flatten primer?

[ar15barrels]

What's your opinion of the picture, is this a flattened primer, a slightly flattened primer or a dangerously flatten primer?

My opinion is that you can't rely on traditional pressure signs in large case-head AR conversions because traditional pressure signs arrive much after you are in unsafe bolt-thrust areas.

In most guns, you can use pressure signs because the action was natively designed for the cartridge at full pressure (55k to 62K) levels.
The native cartridge of the AR is the 223/5.56 with a 0.38" boltface.
AR wildcats with larger boltfaces will require lower pressure levels to keep thrust and ultimately bolt-life in check.

You should not get pressure signs at 35,000 PSI, or even at 45,000 PSI, but you certainly would at 60,000 PSI.
This is fine when the original design is still safe to these levels of pressure/thrust.

Remember that chamber pressure does NOT correlate to thrust until boltface size is also factored into the equation.
Pressure levels that are safe in smaller cases will not necessarily be safe when boltface size increases.
Reading pressure signs on primers is helping to determine pressure, not thrust.

Play with my thrust calculator and see what calculated thrust you are creating by elevating chamber pressures until you get pressure signs.

If you want to try to read lower pressures from a primer, try pistol primers.
They have a softer cup and show signs at much lower pressures.

[Slash2]

Play with my thrust calculator and see what calculated thrust you are creating by elevating chamber pressures until you get pressure signs.

I'm not sure I'm clear on this, but according to your calculator with a .471 base dia. I hit 7001 lbs of backthrust at 40,200 PSI. You are saying that I shouldn't start to see signs of pressure (flattening of the primer) until well above 45,000 PSI, so when I start to see signs then I'm probably in 55,000 PSI range, which is potentially generating 9578 lbs of backthrust?

[ar15barrels]

so when I start to see signs then I'm probably in 55,000 PSI range, which is potentially generating 9578 lbs of backthrust?

Yes.
That's what I am saying.



Now re-run the numbers using the major diameter of the case as the case-head size.
The rebated 284 parent case does NOT lower the thrust.
Thrust should be calculated by the major diameter of 0.5" which will increase your 55Kpsi thrust level results even more.

[ar15barrels]

Re-posting this because it ties in with the above post:

Commonly used maximum bolt thrust loading on AR-15's is 7000 lbs.
A 223 at 55,000 PSI makes about 6300 lbs of thrust.
7.62x39, 6.5 Grenade (grendel) and 6.8 SPC all run right at 7000 lbs.
Using the same formula, a 450 Bushmaster at 60,000 PSI makes 11,775 lbs of thrust.

As a comparison, a 308/7.62x51 makes 10,500 lbs of thrust.
308's use a bigger bolt for obvious reasons...

If the AR-15 could run all day long with 10,000lbs of thrust loading, why did they bother making the bolt bigger on the AR-10?

[Slash2]

Point well taken. I'm going to go with the advice of the bench rest guy that was in the stall next to mine. "If you're accurate at 2100 fps why do you need to go any faster? A bullet that big, going that fast, is going to kill anything that can stand up in front of you as it is!

If I find a hog that can shrug off one these rounds, then maybe I'll worry about more power! For now, I'm going give my gun, and my shoulder, a break and stick to the sane range.

[Wildcatter]

My opinion is that you can't rely on traditional pressure signs in large case-head AR conversions because traditional pressure signs arrive much after you are in unsafe bolt-thrust areas.

In most guns, you can use pressure signs because the action was natively designed for the cartridge at full pressure (55k to 62K) levels.
The native cartridge of the AR is the 223/5.56 with a 0.38" boltface.
AR wildcats with larger boltfaces will require lower pressure levels to keep thrust and ultimately bolt-life in check.

You should not get pressure signs at 35,000 PSI, or even at 45,000 PSI, but you certainly would at 60,000 PSI.
This is fine when the original design is still safe to these levels of pressure/thrust.

Remember that chamber pressure does NOT correlate to thrust until boltface size is also factored into the equation.
Pressure levels that are safe in smaller cases will not necessarily be safe when boltface size increases.
Reading pressure signs on primers is helping to determine pressure, not thrust.

Play with my thrust calculator and see what calculated thrust you are creating by elevating chamber pressures until you get pressure signs.

If you want to try to read lower pressures from a primer, try pistol primers.
They have a softer cup and show signs at much lower pressures.

I will souvenir you some once fired brass, goes the same for several other gents that are trying to do some load development. This is all in its infancy, that is, as far as the public domain is concerned and if we can get enough guys safely loading to the near potential, of this cartridge, you will see great things in the future.

[Wildcatter]

I had a chance to test some loads today. All were loaded with Hornady XTP/MAG 240 HP.

Mostly I shot 36.5 grs of H110 with an AOL of 2.1 and 2.15". The 2.1" cronied at around 2150fps with the 2.15AOL running 50fps less. Both grouped around 1.5 to 2" @ 100yds.

Next I wanted to try some heavier loads. I started at 38.5 grs H110 and stepped up .5 grs a round till I was at 48 grs. All at 2.15 AOL

The fps stared at 2150. I didn't see much flattening of the primers until around 43.5 grs. the velocity was 2475 fps at that point. I fired 3 more and then decided to quit at 45 grs and 2550 fps. The primer was getting pretty flat but there was no sign of a crater.

The recoil at this point was substantial but the bullets were making a nice straight line, one above the next, over the x.

What's your opinion of the picture, is this a flattened primer, a slightly flattened primer or a dangerously flatten primer?

[Wildcatter]

PSS.. Make sure that during your work-up faze that you keep your bolt face cleen, take a tooth brush with you. The crud build-up will mark the case head and can be prematurely mistaken for bolt thrust/pressure problems.

[Wildcatter]

Re-posting this because it ties in with the above post:



If the AR-15 could run all day long with 10,000lbs of thrust loading, why did they bother making the bolt bigger on the AR-10?

Randall, with all due respect, the .308 acutally does have very much more bolt thrust than the 450b on an AR bolt, remember that very real cohesion thing. Example, you could rechamber the 7.62x39 AR to a .308 chamber, open the bolt head slightly, single load the weapon and in short order break the AR bolt, not so with the 450b @ 50,000++psi.

[ar15barrels]

Brush-up on pressure signs as they appear in 5.56 AR's...

[Wildcatter]

I guess you can't sleep either, ah?...Great pictures! What do you think the pressues are when a 5.56 case has major ejector marks and blows its' primer, like you show? I'd dare say we'd be into the 70,000++ psi realm and did these loads break there bolts? Maybe, maybe not and overtime, probably. Bottom line, this is what I was talking about for excessive pressure signs. The Slash loads don't even approach the ones you are showing and have no visiable bolt thrush problems. Now Slash has a visiable rum line to stay away from, thanks Randall, you're the greatest.

[ar15barrels]

I guess you can't sleep either, ah?...Great pictures! What do you think the pressues are when a 5.56 case has major ejector marks and blows its' primer, like you show? I'd dare say we'd be into the 70,000++ psi realm and did these loads break there bolts? Maybe, maybe not and overtime, probably. Bottom line, this is what I was talking about for excessive pressure signs. The Slash loads don't even approach the ones you are showing and have no visiable bolt thrush problems. Now Slash has a visiable rum line to stay away from, thanks Randall, you're the greatest.

Brass hardness is a factor so pressure levels can't often be determined by these signs.
In proper brass, ejector and extractor flow is probably around 65K to 70K
Primers probably go around 70K to 75K

Proof loads for 5.56 run 70K and don't pop primers, but I believe that black pill loads are made up in crimped LC brass.

Those pictures above are from Remington factory ammo with soft brass.
No bolts were broken in the firing of that ammo.