Quandry ... how to handle max velocity AR-15

T

Tommy Gun Sucker

Inactive
Hi all,
Been reloading a lot of years but never ran into this. I have an AR varmint gun with 24 inch barrel. Bullet manufacturer states their test barrel is 20". To my knowledge, you get about 50 fps more per inch of barrel length. My gun does appear to be shooting about 200 fps faster than the book for a given charge of Varget.

Normally I would say yea! in a bolt gun but for an AR, I don't want to over-pressure the gas system. Does the pressure the gas port sees remain about the same while just bullet velocity increases or does the gas port also see additional pressure?

I worked up to just under max charge and the bolt bent the case rim a bit during extraction. Don't know if it is the pressure blowing the bolt back too fast or another sign of over pressure causing high friction extraction. Any help is appreciated.
 
Twist rate and bullet bearing surface also play a role in velocity.....Are you shooting a rifle with the same twist as the test barrel?......Are you shooting the exact brand of bullets used in the testing or just the same weight?

Those are just a few other things that have a decent effect on velocity other than the major players like barrel length (which is most likely the reason for the higher velocity). There are many other minor things involved too.

If you are 100% sure that your charge weight is correct, you aren't over the book max and aren't seeing pressure signs in your primers or case heads I wouldn't be too concerned (but it sounds like the extraction issue is telling you something though, over gassed?)......Some guns just shoot faster than others.

What do you mean by "bolt" bending the case rim?........I suspect you mean "extractor"

EDIT:......Oh yeah and whether or not you have a 223, 5.56 or Wylde chamber can make a pretty good difference in pressure too....
 
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Tommy Gun Sucker wrtoe:
To my knowledge, you get about 50 fps more per inch of barrel length.
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Where does that "knowledge" come from?

Since the original AR had a 20 inch barrel and the carbine length versions have a 16 inch barrel, and so loads are being optomized for those barrel lengths, not 24 inches. A load optimized for a 20 inch barrel may see ZERO increase when fired out of a longer barrel. In fact, with the additional friction from the longer barrel, the velocity could actually be LOWER.

What bullet are you shooting?
 
For the same barrel, until you get out to 30+ inches, velocity will increase.

Most data says about 25 fps per inch, but that is average.

You barrel is a combination of a bit longer and maybe less friction.

The ported pressure will not drop any velocity.

Ergo, you want to load for an AR not a bolt action if you plan on shooting the rounds in an AR. Port location and the cartridge charge are a balanced aspect.

A short or long stroke piston if it has the adjustment can be tuned around that (RA XCR for one).
 
Thanks for the replies. The particulars ... yes, I meant extractor bent rim. Source of load data: Sierra Infinity Suite load data (Sierra data book) for AR-15 loads. They use Colt's AR-15A2 HBAR with 20" barrel 1:7 twist. My gun, Rock River Varmint 24" bbl, 1:8 twist with Wylde chamber.

Sierra's Components My Components
Case -Federal Case - Federal
Trim length 1.750 Trimmed length 1.750
Primer Rem 7 1/2 Primer Rem 7 1/2
Bullet Sierra 69 gr HPBT MK Sierra 69 gr MK
Powder Varget Varget
Min chg & vel 23.3 & 2500 24.0 gr & 2827 (for 24.1 gr, Sierra lists 2600 fps)
Max chg & vel 26.1 & 2850 25.6 gr & 3021 (well under Sierra max chg...per Sierra
should be about 2780)

Using Dillon electronic scale ... it reads within 0.2 grains of my old balance beam scale at 25.6 grains. I'll believe the Dillon even though my only check weight is 50 grams and reads 49.99.

As far as where 50 fps per inch in velocity comes from ... over 45 years reloading experience where all other factors are the same. I realize that number falls apart for SBRs and where the powder doesn't well match the barrel length. I just don't have the resources to have custom barrels built with identical twist, identical tooling and cut to different lengths. If you know of a reputable site on the web where that has been done, I can always learn.

I would love to have someone who can show me pressure profiles in the barrel up to the point the bullet passes the gas port 20" vs 24" at Varget pressure levels.

Sorry, don't have an adjustable gas block or I would simply adjust to match the loads. The round with the bent rim showed no other signs of over pressure. I suppose it could be a very old piece of brass but counting extraction marks, looked like it had only been reloaded 4 or 5 times.
 
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Velocity doesn't always equal pressure and pressure doesn't always equal velocity.

If you had a barrel long enoug the bullet would take off in it and at some point slow and then stop.

If you put a super fast powder in a large enough quantity to blow up the reciever of the firearm you would have lots more operating pressure, that might not have even been contained long enough for the bullet to exit the barrel.

The longer barrel does allow the pressure that is there to push on the bullet longer, before the barrel ends and that pressure turns into noise.

To your question, the pressure at the gas port on a 7.5" .223 barrel is quite high, much higher than on a 24" but lucky for us that was taken into account when they were building the barrels and drilling the gas ports (diameter) or one can get adjustable gas blocks and "tune" the rifle. So enjoy, for once your "book data" loads that are faster than the book, just don't try and get that velocity from a 7.5".
 
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The point I was trying to make is that if you need to back that load down.

And how did you determine the 200 fps?

As noted on many posts, its the gun not the load that is key driver.

Unfortunately in a semi auto its very difficult to detect over pressure until something lets go.

As its a non regulated gas port, any higher pressure is going to impact the action.

Sticky case and it will attempt to extract that round regardless.

A DGI gas gun is intended to function in a specific range. Your 4 extra inches might get you 100 fps. If the 200 fps is valid then that gun is pushing them out faster.
 
Well IMHO , the 24"barrel has a much longer dwell time if you are running a rifle length gas system . This will cause pressures to stay high in the chamber longer resulting in the case still being stuck to the chamber wall during extraction as evident by your bent case rims .

An adjustable gas block should solve the problem by reducing the gas allowed through the gas system .

The other thing that may help is using a faster burn rate powder . Varget is slow-ish and "may" be keeping pressures higher then you want as the bullet is in the dwell time zone . A faster powder how ever will almost certainly reduce your over all velocity .
 
"...at Varget pressure levels..." Powders don't have pressure levels. Cartridges do. Powders have burn rates. SAAMI max pressure for .223 is 55,000 PSI. Load manuals and ammo makers comply with SAAMI specs.
Anyway, according to Hodgdon, the MAX load of Varget for a 69 grain bullet is 26.0(C) at 50,200 CUP at 3,010 FPS out of a 1 in 12, 24" barrel(that length is odd enough too). There will always be differences between manuals. All data in all manuals is safe to use.
Your barrel and twist are different than the test barrel. You'll get difference velocities with the same pressures.
What is your exact load? The Book velocities don't matter as Sierra used a 20" barrel and you don't.
The extractor thing appears to be related to what buffer you have. No such thing in M1's or M14's.
"...50 fps more per inch of barrel length..." Gets argued about regularly. Long ago, one of the gun rag writers(don't remember who. Or what gun rag.) conducted an experiment by cutting 1" off a barrel from factory length and checking velocity with a chronograph(long before everybody could afford one). Don't recall if the twist was even mentioned. (Probably not as it was long ago and such things weren't discussed then.). His results were 100 FPS per inch.
 
I think you are overthinking this a bit. OK, you have 4 more inches of barrel beyond the gas port that the standard rifle. So, this is going to get you higher velocity than the standard rifle, with the same loads.

MAYBE you are "over pressure" at the gas port, maybe not. Think about it, the fired round isn't producing any more pressure because of your longer barrel, it produces higher velocity with the existing pressure, because that pressure can push on the bullet longer (over more time) because your barrel is longer.

And, even IF you are slightly too high port pressure, the simple answer is just back you load down a little.

I worked up to just under max charge and the bolt bent the case rim a bit during extraction.
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Every gun and ammo combination is slightly different, and just because a book or the internet says load "xyx" is max, doesn't mean that it is max in YOUR gun with YOUR ammo. It means it was the max that they got to with their gun, and their ammo. Your rifle's safe working max load may be under or over what they got. And, if you are bending the case rims during the extraction cycle, you are at max for that combination of components, in that rifle, no matter what any book says.

Reloading data are guidelines, not immutable laws, and most things fall close to the middle of the bell curve, which is why they are useful guideless, but there are examples that fall at each end of the curve, and only shooting and checking results will show which is which.

bending the case rim means the rifle tried to extract the case before pressure in the chamber had dropped enough to allow it. A fixed gas system has its limitations. And a direct impingement system (the standard AR setup) has even more limitations on what it can, and can't do well. It's a balancing act, and when you introduce handloads with potential pressure curves outside of what the rifle is build for, you get issues.
 
Powders don't have pressure levels. Cartridges do.
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That is getting pretty picky. I clearly understood what he what he was conveying.

If someone says Cat dozer and its a John Deer I can understand he is talking about the formally ubiquitous original Cat built dozer (technically a form of a continuous track laying tractor dirt mover using a metal plate known as a blade (though more technically Best/Holt was there first)

A barrel does have pressure in it as a result of the gas release from the powder converting the powder into energy resulting in pressure.

And that is what we are talking about.

Relevant is the need to cut back the load. Straying into Dragon territory (a mythological beast of varying descriptions that are considered dangerous and encouraging people not to go into their caves etc)
 
Ok guys ... thank you all for your comments. I pretty much know I'm not going to use the load the bent the rim. I would normally use the "how far it ejects" as a gauge to over pressure in the port like I do for my Garand and M1As. However, in this case I was on a public range with screen between stations and couldn't tell what was happening. FITASC, thank you for the thought ... I was shooting across my chronograph when this happened.

I think I'll just back down a couple tenths on powder and go to the desert to take a look at case ejection distance. All of my ARs tend to pile the brass into a fairly small area. If it's throwing brass 5 feet farther than my other ARs, I'll know I'm hitting the port too hard.

Thanks again and I think we can put this post to sleep.
 
FWIW,I started loading Varget behind a 69 gr MK before there was published data. 1 in 9 16 in Hbar. I used my chrono and incremental velocity gain to find my own max load.I also contacted Hogdon and Nosler. At that time,they would snailmail data. We all agreed on a 25 to 25.5 gr max.
I do not have the rifle anymore.I forget if it was 2780 or 2870. One of those.
Dependably accurate. IMO,the go-to 69 gr load.
IMO,the solution to a 24 in bbl is a band saw ,etc.
I'd agree its likely a dwell time vs peak pressure issue.In both cases unlocking occurs at the same time. The 20in bbl vents off 4 in sooner.
Just my opinion. I'm not the expert. The .223/5.56 brass is limited in the amount of brass around the primer pocket. The price of hot rodding a .223/5.56 is scrapping your brass to loose primer pockets.
I understand your extractor is pulling your rims.IMO,that is not the definitive story. It may be that an adj gas block or heavier buffer will get you past the pulled rim.
If you can get 4 to 6 loadings and keep tight primer pockets the pressure of your loads is OK,IMO.
You just have too much gas too early.But,my opinion is worth what you paid for it.
 
I had more trouble tuning a 24" barreled AR than with any of the more common 20" barrels. Final solution was an adjustable gas block and heavier buffer. That fixed all the issues and would be my recommendation.
BTW the "50 fps per inch " is relevant WHEN REDUCING BARREL LENGTH and may not match the + velocity WHEN INCREASING BARREL LENGTH.
 
50 fps per inch " is relevant WHEN REDUCING BARREL LENGTH and may not match the + velocity WHEN INCREASING BARREL LENGTH.
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OP is happy but this is interesting.

First the data I see says more like 25 fpi.

Second, if you add an inch of barrel vs taking off an inch that changes things?

I don't agree or buy that.

Assuming all else is equal (not likely but close enough)\

I buy a 30 inch barrel, and it does 3000 fps

I then cut off 10 inches (for a clear change) and it does 2750 fps

I buy another 30 9nch barrel (because I can't weld on) and its not going to do very close to 3000 fps?
 
Something to chew on.

Rather than pressure and dwell, think of it as overall energy.

Say you do 100,000 psi for 2 seconds

Then you do 100,000 psi for 4 seconds.

While your PSI is the same, the energy you are putting into your action is doubled.

While I know some of the laws are non linear (aircraft speed vs engine hp and water flow and pressure ) I don't know about PSI and duration.
 
Logic tells me that the increase or decrease of velocity due to longer or shorter bbl is a function of powder burn rate. If the powder is slow burning, an increase in bbl length is expected. Shortening a bbl with the same slow powder will reduce velocity. I always try to pick a powder in the lower third of burn rates by the book.

As part of a smokeless gunpowder photo series I am doing, I wanted to create a burn-rate crossreference table for each type of powder. That effort was very much more difficult than I thought. I never found two burn-rate charts that listed the powders in the same order. In one case, two major sources disagreed on one powder by 20 powders! Western Powders method offered the best explanation ... some powders are just too close to differentiate. They use a method where powders are placed in groups with similar burn rates. I chose to follow their methodology.

If I had my druthers, I would choose a powder that was totally burned as the bullet exits the muzzly. I really can;t tell that other than by shooting at night. If I get a large flame, the powder is still burning. So I won't worry too much. I backed down and found a load of Varget that consistently gets me 0.61" groups average at 100 with 0.501" min and 0.82 max (10 each 3-shot strings). I'll tweak this a bit and see if it gets better .
 
I do not have QuickLoad. You have to enter parameters like case volume,barrel length,etc but it models predicted pressures,velocity,and powder % burn

Part of what you are up against with your burn rate charts is burn rates are pressure dependent.

Two powders may be nearly identical in burn rate at 50,000 psi,yet one may begin to spike at 60,000 psi.

One may be relatively linear and useful from 25,000 through 65,000..
The other might be a mess below 50,000.

Gene Stoner put the gas port where he did for a 20 in bbl and 55 gr bullet.

Gas port pressure may be higher with slower powder and heavier bullets.

Add 4 in barrel beyond the 20 in,and the case is obturated and pressurized for a longer time after the gas says "Unlock and extract" at the same gas tube length.
That's why your extractor is pulling the rim.Your pressure peak may be fine. Bullet leaving the muzzle at 20 in is a big vent.

I'll say again,.223 brass is slightly weak around the primer pocket. If you dance to the edge,the pockets get loose.

I don't like scrapping my brass early. I load pressures that keep my primer pockets good for at least 4 loadings..and more is better.Maybe 6. Past that,maybe stretch rings show up.
 
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As part of a smokeless gunpowder photo series I am doing, I wanted to create a burn-rate crossreference table for each type of powder. That effort was very much more difficult than I thought. I never found two burn-rate charts that listed the powders in the same order. In one case, two major sources disagreed on one powder by 20 powders!
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I've always looked at the manufactures burn rate chart . Then use that as a general point . Trying to find a difference between #72 & 79 there's not likely enough difference . How ever IMHO if a manufacture says a specific powder they make is slow for the caliber then I believe it .

If you look here at Hodgdon's burn rate chart http://bulletin.accurateshooter.com/2016/01/updated-hodgdon-and-imr-relative-burn-rate-chart/ you will see that the spectrum of powders that work for the 223/5.56 is numbers 72-ish through 105 . By them putting Varget at 101 tells me they ( hodgden ) think it's a slow powder for the caliber . Now that would be a faster powder for 300 WM . My point there is not to try to figure out the powders specific burn rate but rather understand it's general place on the chart . Varget is a slow powder for 223/5.56 .

Unclenick can speak to this better but it's my understanding a slower for caliber powder will peak further down the bore and sustain higher pressures ( not peak pressure ) for a longer duration then a faster powder . That's where my suggestion came from when suggesting a faster powder "may" help . An adjustable gas block is still my first recommendation .
 
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