5.56 velocity mystery, need help

[bigedp51]

Question, I do not have a chronograph so this is why I'm asking this question.

Can a flash suppressor and a slow burning powder cause any malfunction in the chronographs readings with more muzzle blast and disturbed airflow over the screens.

 

[Mr. Bullet]

Ok, Ed, Kevin and Jo have convinced me that I need to perform a new test, trying to control for each part of the formula.

I'll use the same batch of primers, a new jug of W748, and will take two rifles this time, a 16" and a 20". Will also put new a new battery in my chrono and get it set up and working perfectly with factory loads before starting testing.

Should be able to do this by late tomorrow.

 

[KevinThomas]

BigEd,

Not usually a problem when dealing with supersonic loads, but I'm sure it can happen. First screen placed a bit too close to the muzzle, weird lighting conditions, that sort of thing can cause problems. For lack of a better term, they can be tweaky at times. Blast shields go a ways towards reducing this sort of thing, but there always needs to be some judgment exercised in using a chronograph as well. If it's telling you something too good to be true . . . it probably isn't. On the flip side, things like the OPs issues should raise red warning flags. The commercial models that are permanently mounted and using IR screens instead of photo sensitive sensors make life SO much easier.

 

[DRNewcomb]

My W.A.G. is that the OP grabbed a can of some really slow powder by mistake. I give up. That's all I can think of.

 

[mytwo60]

I don't know what you did but I would document it and sell it for some serious cash. I mean if you can get a subsonic load and have it cycle out of an AR that is money. With a 55grn bullet to boot!

 

[fredhorace77]

Just some thoughts here. do you tumble your brass with corn cob media. and if so you may have had a kernel stuck in the primer flash hole. like a previous post stated you should be able to look down the barrel and see if there's unburnt powder

 

[Pres100]

Were the charges actually hand weighed or thrown on a progressive? Have you pulled any bullets and actually reweighed the charge?

 

[bigedp51]

Just some thoughts here. do you tumble your brass with corn cob media. and if so you may have had a kernel stuck in the primer flash hole. like a previous post stated you should be able to look down the barrel and see if there's unburnt powder

Real men don't worry about tumbling media in the flash holes, that's why military rifles have chrome lined bores. Besides anything extra that hits from a AR mouse gun barrel has got to help with knock down power. : (I was told the pins hit sideways and tumble)

 

[searcher]

I did a few boners in the past that led to what you describe. I had cleaned my cases in a sonic cleaner and left them to dry. I loaded them a few days later, thinking they were completely dry inside (the couple of samples I looked at "appeared" bone dry). I fired them several weeks later and ran into exactly what you describe. While I wasn't running them over a chrono, you could tell something was terribly amiss when they fired. Very anemic and underpowered. For fear of having a bullet jam in my bore, I stopped after a few trounds. I pulled the bullets in the rest of that batch and noticed the powder was clumped in several of the cases. It was NOT like that when it went into the case as each charge was weighed on a scale. I surmised there was excessive moisture in my cases, possibly even droplets of water - enough to cause the powder to clump. I used the same jug of powder and lot of primers before and after with no such problems.

I also had left pistol powder in my dispenser for a few months (I never do that....) and decided it should be O.K. as a straight-walled pistol cartridge shouldn't pose any real problems (.44 spec.). I also don't live in a particularly humid environment and it had not rained or anything since leaving the powder there. I was shooting Red Dot and you would think I was shooting black powder! Black crap on my hands, white clouds of smoke, the works. Half the powder blew out unburned.

The moral to both lessons is keep your powder dry! I am betting moisture was your problem - whether sourced from your powder, brass or both. Overzealous lubing could contaminate your powder - but you'd have to work extra hard to contaminate it that much. I've shot plenty of primers left to this same environment for 20 years and never had a problem that was moisture-induced.

 

[Old Navy]

BigEd said:First screen placed a bit too close to the muzzle. This I did and the readings were all over the place. Mostly low. Backing them up a few feet will not screw the readings. Later! Frank

 

[Mr. Bullet]

Searcher and Frank, I'm thinking it must be one of the two - moisture or chrono too close. I would like to think I just had the chrono too close, but then why would factory rounds I tested at the same time read normally? Also, the report and the POI were low. I'm leaning toward damp powder, just not sure how it could have happened.

will both fix it with controlling for variables, and try to reproduce the results.

 

[Mr. Bullet]

and to add to the confusion, i did disassemble 5 of the 8 rounds - no light loads and no powder clumping.

 

[Beau]

My psychic prediction is that you have powder ignition problems and are really getting about 2,000 fps out of the squibs (because you said they barely cycled the action sometimes). The ultra-low velocities are being displayed because the chrono is measuring the velocity of the ejected unburnt powder.

 

[bigedp51]

My psychic prediction is that you have powder ignition problems and are really getting about 2,000 fps out of the squibs (because you said they barely cycled the action sometimes). The ultra-low velocities are being displayed because the chrono is measuring the velocity of the ejected unburnt powder.

At a velocity of 2000 fps the port pressure would be too low to cycle the action, a mid length gas system needs well over 20,000 psi port pressure to cycle the action. And 2000 fps would be equivalent to having 18.5 grains of 748 in the case.

 

[Webster]

If you were getting 500 fps you would feel a very large reduction in recoil. I don't have sun glint shields on my chrono. Recently after about 3:30 I was getting funny readings. I don't think it was misalignment of the sensors to the bullet path. I think it was on the Oehler website? It says that if the bullet passes over the sensor with the sun at a low angle, the sun will reflect off of the bottom side of the bullet into the sensor. The sensor works by the bullet causing a shadow or reduction in light as it passes over. If the sun reflects off of the bottom of the bullet into the sensor it causes the opposite to occur. It's something to consider?

 

[Mr. Bullet]

Well, I didn't quite figure out what caused it, but I did correct it. Went out with some loads of same primers - Win, decreased the load to 27.3 gr of Win 748, and got the following - avg V of 2945 in the 16", and avg V of 3015 in the 20". I could try to recreate the problem, but that would take quite of bit of trial and error - maybe take 5 or more trials. I think I know what it was though. My reloading bench is about 5 feet from the door of my bathroom, which does not have a vent. There's shower steam pouring out of there 2x a day, (and shit steam at least 3x). I'm going to keep the powder measurer covered with a ziplock as long as I'm keeping powder in it, but will not make a habit of leaving powder in there for more than a couple of days.

Thank you all for your help and ideas - wish I could say for sure who was right. I think it was probably both moist powder, and chronograph too close the muzzle, but will probably never know for sure.

BTW, harvested 3 muflon mountain sheep today on Mauna Kea with my 30-06, using up the last of my factory loads - Hornady 165 gr superformance sst. Ranges were from 150 to 200. The rounds are accurate but dump energy too quickly. The exit wounds on the thin bodied sheep were avg 6" in diameter. It was ridiculous - looked like a grenade went off in them.

Thanks again, aloha.