Does bullet runout matter on a gas fed gun?

[hounddawg]

The guy strikes me as good people I am sure he will make things right. The muzzle brake thing does make sense on the physics end but dayumm that is sexxy brake. I'll just run a thread protector on this gun and put that bad boy on a bolt gun if it turns out to be the culprit.

[Unclenick]

The brake will start to bleed pressure down before the bullet is completely clear of the end of the brake. This is so the gas in the barrel spends energy going sideways and does not act like a rocket engine pushing straight back after the bullet clears the muzzle, as happens with no muzzle brake. Rocket effect is a significant portion of overall recoil in some cartridges, especially large case capacity overbore cartridges. I know of no way the shape of the brake would drop pressure any faster than will happen with your thread protector on. The limiting constriction is the bore size. Drawing the bore pressure down faster would require an outside source of energy. Unless the added recoil from lack of a brake assists the bolt going to the rear, I don't see how removal would improve cycling. Let us know what happens.

[hounddawg]

@UncleNick

I am thinking of the lift effect of a airplanes wing, just theory but can find no binding in the bolt and everything else checked out. Jet pumps, pitot tubes, and 2 stroke engine scavenging effect operate on the same gas theory. I think the design of the brake is causing low pressure in the last few inches of barrel. It almost cycles, just the bolt is not coming back far enough to pick up the next round. Ejection is fine

Gas port and gas block alignment is borderline but the port looked like it was fully exposed, gas tube was clear with no debris. Gas port was also checked with a pin gage. Seal at the gas tube/gas key checked is ok judging from the carbon deposits.

Small companies like this rely on word of mouth, Criterion did a wonderful job a couple of years ago with a barrel that I could not get to shoot. Replaced it with that .223 barrel I am always posting bragging targets from. Gut feeling is Sanders will do the same. Worst case scenario is I would pop a Criterion barrel on this rifle but I really don't think it would ever come to that. The entire upper just screams quality in the fit and finish. As good if not better than my White Oak upper. By this time next week it will be fixed or the upper will be on it's way back for a new barrel

[zeke]

It appears the there is not enough gas from the 6.5 loads to cycle the buffer/spring that is set up for the 223 gas/barrel length, and possibly the gas port size is too small. Other things to consider trying are changing the buffer weight/spring lighter to see if will cycle, or using a powder/load combo that produces more gas.

Have several larger bore AR's, and if practical prefer to use reduced gas volume rather heavier buffer/spring, so long as the combo is reliable. Overly large gas ports was common problem when more manufacturers started making larger bore ar's, which may be why adj gas blocks became so popular.

[hounddawg]

Thanks, that is good to know Zeke, I am a novice on gas operated rifles so any suggestions are welcome. I suggested a lightening the buffer tube weight but the manufacturer disagrees. The buffer tube is carbine length and weighs 2.95 oz and spring length is 10.5 inches. The rife cycles fine with either of my 20 inch barreled .223 uppers cycles even with powder puff plinking loads

[Unclenick]

Assuming the same barrel length, when you go to a larger caliber you are increasing the expansion ratio of the gun. Unless case capacity is increased to bring the expansion ratio back in line with a same-barrel-length .223, then you can expect pressure will drop more per inch of bullet travel than it does with the .223. Thus, if the gas port is the same size as the .223 gas port, it will have to be located further to the rear to get the same magnitude gas impulse. If it is to be kept at the same location it will need to be enlarged to allow the lower pressure to produce the same gas impulse back to the bolt carrier. Note that unless you use more case capacity, the bigger bullet is moving slower, so its dwell time between passing the gas port and exiting the muzzle is greater and that partly makes up for the drop in pressure, just not usually all of it.

Regarding the brake, an open muzzle vents gas as fast as it can be vented and therefore drops pressure faster than it will drop by any other means. The brake will introduce resistance to exhausting because the gas has to be turned to the sides to get out. That added resistance to the gas escape prevents pressure dropping as quickly as it will with a simple open muzzle. As I already mentioned, getting gas to empty the bore faster than an open muzzle and without enlarging the bore will require adding energy to the system that the powder hasn't already provided, and I don't know where that would come from. I am going to be hard to convince a brake is generating free energy. But I'll always entertain arguments to the contrary.

[hounddawg]

Dunno Nick, guess I will find out Tuesday or Wednesday. I am taking that upper and a .223 upper. A thread protector, the original muzzle brake and Precision Armamant M11A2-7 brake along with 10 boxes of Wolf ammo and a few handloads in case it cycles also a variety of .223 factory and handloads and going to make it a AR kind of day

At this point my gut tells me that the gas port in the barrel is too small, either it cycles with the Wolf Tuesday or it heads back to Florida for a new barrel

edit - after giving it some thought I can see where the brake could cause an issue. Maybe the reduced recoil is vs full recoil is affecting the inertia of th ebuffer tube slightly. The brake definitely changes the way the rifle reacts and the gas flow so it's something worth looking into.

[Unclenick]

Yeah, the inertia idea occurred to me, too, but I wouldn't want to rely on a weapon running that close to the edge. I'm guessing you are looking at reaming the gas port a little.

Let's see what happens at the range.

[hounddawg]

I won't make any call one way or the other. after watching some bullet shockwave videos it's pretty obvious a brake does cause significant changes to the gas flow and pressure waves behind the bullet. Will Quickload model the back pressure curve in a barrel with and without a brake after the bullet exits?

I pulled the bolt apart cleaned and lubed it yesterday also. It ejects great, just won't chamber the next round. It sounds right also, I had a soft cycling .223 that sounded similar and was cured with 20 rounds of some really dirty ammo after soft cycling about 200 rounds of Varget handloads. I think we are just talking a fraction of an inch here

After the next range session if I cannot get it to cycle or group I will test Sanders CS. He sounds pretty like a stand up guy on the phone so I am optimistic it will be cured one way or the other.

[Unclenick]

QuickLOAD does not model brakes. Its recoil calculator does split out rocket effect (QL calls it aftereffect) on recoil from recoil due to equal and opposite reaction to accelerating the bullet, so if you had an ideal brake, you could just subtract out the aftereffect to see the recoil difference. But if you have a brake that directs gas jets up or back, it won't account for their effet.

If you followed a rifle bullet down the tube, you would find the pressure at its base was a couple of three thousand psi lower than is at the breech. This pressure drop is due to the gas having to be accelerated to keep up with the bullet. At the muzzle, if the muzzle is open, as the bullet just clears it obstructs the flow of gas pushing on its base and can actually pick up two or three percent of its peak velocity from that jet of gas on its base. Once it is more that about 11 calibers away, the interaction with the muzzle becomes ballistically insigificant. Pressure in the barrel will then have dropped first at the muzzle and the drop will work its way back toward the breech. That results in a delay before the breech knows the bullet has cleared the muzzle and the pressure in the case starts to drop any faster than it had been as the bullet streaked down the tube.

Because the brake is a constriction, it will slow the pressure drop in the barrel a little, but I don't expect it to be much. I don't expect there to be enough change of resistance to make the gas column in the barrel bounce back and forth as a pressure wave. Such waves do exist in artillery, but IIRC, there is a minimum barrel length for that to occur that is something on the order of six feet and the shell still has to be in the bore for the wave to reflect off of.

You'll have to link me to the videos you are looking at for me to guess at the mechanism of what you are seeing.

[hounddawg]

Because the brake is a constriction, it will slow the pressure drop in the barrel a little, but I don't expect it to be much. I don't expect there to be enough change of resistance to make the gas column in the barrel bounce back and forth as a pressure wave

Like I said I won't call it one way or the other. Far fetched but possible if you look at the pressure waves following the bullet.

https://www.youtube.com/watch?v=BPwdlEgLn5Q

If I take the brake off and it immediately cycles we know the brake was the issue, if it starts cycling after 20 or 30 rounds of Wolf there was gas leakage. If the .223 upper on the same lower cycles we know the lower is good. Just a matter of eliminating everything before returning

[armoredman]

I actually had some ammo loaded that had 55gr Hornaday SP that had visibly off run out, and I decided to give it a try anyway. I tried with the same load and good runout, (as far as I could tell, low tech loader here), and through the BREN 805 the results were pretty much the same.

[Bart B.]

Military teams testing 30 caliber match ammo in semiauto match rifles clamped in accuracy cradles learned .003" bullet runout was the maximim for best accuracy.

Note that a given round's bullet runout will vary depending on cartridge reference point locations, case out of round issues and dial indicator touch point on the bullet.

No commercial runout gauge supports and aligns a cartridge the same as it is in the chamber when fired. It helps to know where external forces position the cartridge in the chamber when it fires.

A perfectly straight cartridge headspacing on its shoulder will typically be a tiny bit crooked to the bore axis when fired. Bullet tip can be .001" or more off bore center.

[Unclenick]

This shows how different means of measuring runout will get different results with the same amount of bullet tilt off the cartridge axis.

[cw308]

Kind of feel sorry for the new guy that is just getting started , I've been shooting for many years and understand what all have mentioned . There's only so much you can expect from a production rifle . If your dead set on accuracy a custom job is the only way to go if your going into what I call the deep water . Never to late but wish I started sooner . Keep up the great reading guys . It's easier to carry a #2 pencil when going down range and showing that bughole group.

[Bart B.]

There's only so much you can expect from a production rifle .

Likewise from shooters.

Will everyone shoot a match winning record setting rifle and ammo to the same accuracy level?

Some competitive shooting associations believed all production rifles and ammo allowed had the same accuracy level. That leveled the playing field so the best marksman would always win; so believed the governing body. All competitors were issued an unaltered production rifle and ammo of the same lot.

[44 AMP]

Is the same concentricity required as in a single fed bolt gun round or even magazine fed bolt gun?

what do you guys think?

I think "required" might not be the right word. I think its more important if your rifle, and you, can use it, and that's something only shooting will show.

What I mean is, if you don't do anything special, just take what your dies produce and shoot it, and get 1" groups, then do whatever to make the most consistent ammo with the smallest concentricity variance possible, shoot it in that same rifle and still get 1" groups, then what's the point?

Now, if it makes a significant difference, that's another matter. But you kind of need to know if that happens, or not.

two stories, illustrating extremes,
there's a story about a guy who had a Steyr SSG, (.308 sniper grade rifle) who only got "minute of paper plate" at 100yds, was convinced his barrel was crap, or the whole rifle was, and was about to get rid of it. His buddy says "shoot some of these, first", some Federal Match ammo. SSG turned in MOA groups with those. The ammo the owner had been using was Phillipine milsurp ball (and apparently it wasn't very good)

Other side of the coin, I've got a Rem 600 carbine .308. Was my first deer rifle. I've shot it a lot, it does minute of deer just fine. 1.5-2MOA or so with everything you put in it. Literally, everything. Does 1.5-2MOA with my best handloads, and with "regular" ones. Does the same with factory loads, does the same with Match grade ammo.

Sometimes high quality ammo makes all the difference. Some times, it won't make much, or even any difference at all. Depends on the rifle, and of course, the shooter.

[Bart B.]

Is the same concentricity required as in a single fed bolt gun round or even magazine fed bolt gun?

Yes. If best accuracy with all rounds fired is the objective.

Check out a full magazine to see if the first to last round holds up from earlier round's recoil.

Applies to semiautos, too.

[hounddawg]

just an update - taking muzzle brake off did not fix the cycling issue, good news is Sander's Armory sent me a mailing label and will fix or replace. Good people to deal with

[Unclenick]

Thanks for the follow-up. Let us know how they do and what they say they did to correct it.

[hounddawg]

@Nick- only thing left to do is enlarge the gas port. Easy enough but just to CYA I am going to let him do it. Lower cycled reliably with mild loaded .223's. Hopefully in 2 weeks or so I can do some load development with the barrel. Lots of guys claim some great groups with this cartridge let's see how much is BS. I would like to see better than minute of hog