5.56 velocity mystery, need help

[Mr. Bullet]

I recently took my new 556 test loads to field to get velocities and got incredibly low V, in the range of 450-500 fps. The recipe is mixed brass, Berger 55 gr varmint bullets, Winchester primers, 27.5 gr of W748, COA 2.24. I previously used this same recipe with 27 gr of W748 and was getting great accuracy but only 2900 fps. I verified that it was not my chrono, and was not a single shot anomaly. when I took the remainder of the test rounds home and took them apart, I found that there were no light loads. (shot 3 of 8, disassembled 5). I did a burn test (not terribly scientific) of the suspect powder, but it burned exactly like the new stuff (on a plate, without a pressure container). Tested the primers too (again not terribly scientific) by firing a few and comparing them to a different brand - again nothing obvious. The powder was in my closed measurer for about 2 months. all i can think of is that it must have soaked up some ambient moisture, causing it to burn with less intensity. Maybe burning it outside of a pressure vessel is not a reliable test. I have used W748 for a couple of years in 308, 223, 30-30 and 30-06 and have never had this problem. Any thoughts, theories or answers would be greatly appreciated.

 

[CatShooter]

I recently took my new 556 test loads to field to get velocities and got incredibly low V, in the range of 450-500 fps. The recipe is mixed brass, Berger 55 gr varmint bullets, Winchester primers, 27.5 gr of W748, COA 2.24. I previously used this same recipe with 27 gr of W748 and was getting great accuracy but only 2900 fps. I verified that it was not my chrono, and was not a single shot anomaly. when I took the remainder of the test rounds home and took them apart, I found that there were no light loads. (shot 3 of 8, disassembled 5). I did a burn test (not terribly scientific) of the suspect powder, but it burned exactly like the new stuff (on a plate, without a pressure container). Tested the primers too (again not terribly scientific) by firing a few and comparing them to a different brand - again nothing obvious. The powder was in my closed measurer for about 2 months. all i can think of is that it must have soaked up some ambient moisture, causing it to burn with less intensity. Maybe burning it outside of a pressure vessel is not a reliable test. I have used W748 for a couple of years in 308, 223, 30-30 and 30-06 and have never had this problem. Any thoughts, theories or answers would be greatly appreciated.

Those velocities with that load are impossible.

You have a chronograph glitch.

 

[DRNewcomb]

I recently took my new 556 test loads to field to get velocities and got incredibly low V, in the range of 450-500 fps.

When you say "450-500 fps" do you mean that they were 450-500 fps slower than expected or that the MV was 450-500 fps? I ran a quick ballistics check and at 500 fps the 55 grain bullet would have 69 MOA of drop at 100 yards. They'd have been hitting the ground in front of the target.

 

[bigedp51]

With a carbine length AR15 with 16 inch barrel these velocities are possible (never say never)

27 grains of 748 2992 fps
27.5 grains of 748 3060 fps
(I load 26.5 grains of 748 for my practice AR15 loads)

Cartridge : 5.56 mm NATO - 5.56 x 45 mm
Bullet : .224, 55, Berger M Var #22311
Useable Case Capaci: 27.608 grain H2O = 1.793 cm³
Cartridge O.A.L. L6: 2.240 inch = 56.90 mm
Barrel Length : 16.0 inch = 406.4 mm
Powder : Winchester 748

Predicted data by increasing and decreasing the given charge,
incremented in steps of 2.0% of nominal charge.
CAUTION: Figures exceed maximum and minimum recommended loads !

Step Fill. Charge Vel. Energy Pmax Pmuz Prop.Burnt B_Time
% % Grains fps ft.lbs psi psi % ms

-20.0 79 21.60 2337 667 26612 8580 78.2 1.053
-18.0 81 22.14 2400 703 28421 8927 79.9 1.027
-16.0 83 22.68 2463 741 30363 9271 81.6 1.001
-14.0 85 23.22 2528 780 32442 9610 83.2 0.975
-12.0 87 23.76 2592 821 34675 9945 84.8 0.950
-10.0 89 24.30 2658 863 37073 10272 86.4 0.925
-08.0 91 24.84 2724 906 39648 10591 87.8 0.895
-06.0 93 25.38 2790 951 42418 10900 89.3 0.867
-04.0 95 25.92 2857 997 45389 11197 90.6 0.840
-02.0 97 26.46 2924 1044 48616 11481 91.9 0.814
+00.0 99 27.00 2992 1093 52086 11751 93.1 0.788
+02.0 101 27.54 3060 1143 55835 12005 94.2 0.764 ! Near Maximum !
+04.0 103 28.08 3128 1195 59891 12241 95.2 0.740 ! Near Maximum !
+06.0 104 28.62 3196 1248 64286 12458 96.1 0.717 !DANGEROUS LOAD-DO NOT USE!
+08.0 106 29.16 3265 1302 69058 12654 97.0 0.694 !DANGEROUS LOAD-DO NOT USE!
+10.0 108 29.70 3334 1357 74246 12829 97.7 0.672 !DANGEROUS LOAD-DO NOT USE!

Results caused by ± 10% powder lot-to-lot burning rate variation using nominal charge
Data for burning rate increased by 10% relative to nominal value:
+Ba 99 27.00 3177 1233 63287 12066 98.4 0.724 !DANGEROUS LOAD-DO NOT USE!
Data for burning rate decreased by 10% relative to nominal value:
-Ba 99 27.00 2763 932 42357 10831 83.8 0.868

BARREL LENGTH STUDIES IN 5.56MM NATO WEAPONS
http://sadefensejournal.com/wp/?p=1093

 

[Mr. Bullet]

"Those velocities with that load are impossible.

You have a chronograph glitch."


Nope. I controlled for that. After the first reading of 490 fps, I fired several factory loads with results in the 3000 fps range. Then back to my hand loads - 470 fps, then back to factory - 3000 fps, then to hand loads 480 fps, and again back to factory loads - 3000 fps

 

[Mr. Bullet]

I recently took my new 556 test loads to field to get velocities and got incredibly low V, in the range of 450-500 fps.

When you say "450-500 fps" do you mean that they were 450-500 fps slower than expected or that the MV was 450-500 fps? I ran a quick ballistics check and at 500 fps the 55 grain bullet would have 69 MOA of drop at 100 yards. They'd have been hitting the ground in front of the target.

Yes, 450-500 fps is right, as in crossbow speeds. I was shooting at a target 25 yards away, but mostly just there to get V on this batch so I could develop a better varmint load.

 

[chuckbuster243]

Wow.... I have had my chrono switch to MPS instead of FPS and give me a scare but nothing like this. But I have had rifles with muzzle brakes in shady conditions give me crazy ES (600-1000 fps) and moved the chrony farther out and it went away. I am familair with that load and cannot see how or why its that low.. Try it in another rilfe and chrono before if it was me... Matt

 

[Mr. Bullet]

With a carbine length AR15 with 16 inch barrel these velocities are possible (never say never)

27 grains of 748 2992 fps
27.5 grains of 748 3060 fps
(I load 26.5 grains of 748 for my practice AR15 loads)

Cartridge : 5.56 mm NATO - 5.56 x 45 mm
Bullet : .224, 55, Berger M Var #22311
Useable Case Capaci: 27.608 grain H2O = 1.793 cm³
Cartridge O.A.L. L6: 2.240 inch = 56.90 mm
Barrel Length : 16.0 inch = 406.4 mm
Powder : Winchester 748

Thanks Big Ed, what program did you use to get this? IT seems a lot more accurate than my manufacture's recipe books.

 

[Mr. Bullet]

Maybe I was a little too generous with the case lube? I've only been reloading for a 3 years and have not made every possible mistake yet.

 

[jonbearman]

Just for grins ,how do you clean your cases?I am thinking possibly you have polish you might have poured in to tumble them shiny and some of it got in the cases or they were still wet if you use the stainless pin method.

 

[KevinThomas]

Haven't seen any mention from the OP of what kind of rifle this was, aside form Ed's comments regarding an AR carbine. Quick question for you here, if this was in fact an AR; did it cycle?

 

[jelrod1]

I have to say I am interested in the cause here because I can't see how this is possible either. I'm assuming from the data Biged read it as 4-500 fps low instead of the actual reading?

Can a primer only partially detonate and cause it? Don't even know if that's possible. Did you happen to check for lots of unburned powder after the shot before firing again?

 

[jelrod1]

Haven't seen any mention from the OP of what kind of rifle this was, aside form Ed's comments regarding an AR carbine. Quick question for you here, if this was in fact an AR; did it cycle?

Very good question.

 

[Mr. Bullet]

Haven't seen any mention from the OP of what kind of rifle this was, aside form Ed's comments regarding an AR carbine. Quick question for you here, if this was in fact an AR; did it cycle?

I was using a 16" barrel AR, and yes it did cycle 2 out of 3 times, but barely. there was also a noticeably quieter report. I would have checked for a lodged bullet, but I saw dirt kick up below my target after each shot and got a new chrono reading.

 

[Mr. Bullet]

I have to say I am interested in the cause here because I can't see how this is possible either. I'm assuming from the data Biged read it as 4-500 fps low instead of the actual reading?

Can a primer only partially detonate and cause it? Don't even know if that's possible. Did you happen to check for lots of unburned powder after the shot before firing again?

I'm baffled too, that's why I had to ask if anyone else had ideas. I eliminated everything I can think of with the exception of possilby damp powder - but I have left powder in this measurer for up to 6 months before and never had this problem before. - or too much case lube and not letting it fully dry.

 

[KevinThomas]

You've got some kind of chrono issue at work here, in addition to whatever's up with the charge. No way that a 450-500 fps load is going to cycle an AR, sluggishly or otherwise.

Powder is fairly hygroscopic, and given a very humid environment, it can and will pick up a good degree of moisture. Certainly enough to see differences in velocity. But the numbers you quoted here have to be attributable to something else as well. I would guess that yes, the velocities are significantly higher that what your chrono read, but they are also well under what you should be expecting. Combination of the two, most likely. Are you set up to load in the garage, or some other area that's not climate controlled? Anything indoors should eliminate enough moisture that this shouldn't be a problems with only a few days exposure.

 

[Mr. Bullet]

You've got some kind of chrono issue at work here, in addition to whatever's up with the charge. No way that a 450-500 fps load is going to cycle an AR, sluggishly or otherwise.

Combination of the two, most likely. Are you set up to load in the garage, or some other area that's not climate controlled?

I was surprised it cycled too at these speeds, and maybe my chrono did do something weird. It just seems extremely unlikely that it would make the same mistake 3x when it was performing fine on factory loads. but what did Sherlock say? - something like "when you've eliminated all other possibilities, now matter how unlikely...".

I reload indoors, but we don't exactly have a "modern" house. We're in Hawaii, with 1" single-wall construction on post and pier with no sub-floor (I can see the ground 16" below when I drill a hole in the floor), no insulation anywhere, 70 year old windows, etc etc. (but my gun safe is not moving without some serious demolition!) Unheard of in the mainland, but very common out here.

 

[jo191145]

Interesting. I've seen strange chrono numbers but nothing ever like that.
If it ever happens again crack your receiver and take a look inside the barrel. You should see lots of unburnt powder to let you know whats happening.

 

[bigedp51]

If you are using the same can of powder, the same primers, the same cartridge cases for your 27.0 and the 27.5 load of 748 I would suspect human error reared its ugly head on your second higher pressure load.

Commercial AR15 rifles are over gassed to ensure the bolt cycles with lower pressure commercial ammunition. Your load of 27.5 grains of 748 exceeds the maximum pressure of .223 commercial ammunition of 55,000 psi but is lower than maximum military chamber pressure of 62,000 psi.

Below are the port pressures of the different length gas systems on AR15 rifles



If you used a different batch/lot of primers then I would change primers. A weak primer and 748 ball powder do not like each other. I also use 748 in my 30-30 carbine with a 16 1/2 barrel, and if I do not use a magnum primer I will have unburned powder in the barrel.

Your mid length carbine might like H335 better as it is the standard used by the military in the M16/A4.

 

[KevinThomas]

Well, I've seen chrono's react this way from time to time, especially with the photoscreen types that most consumer chronographs use. It can be as simple as the "glint" problems associated with light angles and reflections, and can show up in certain velocity ranges, and not others. They're weird, get used to that. I got a call from one of the USMC teams once regarding a problem they had with their 45 ACP rounds. They were confused by the fact that they were getting incredibly high velocity, and absolutely nothing in terms of ES or SD. Each and every shot they fired was running 1126 fps, ES of 0, SD of 0. Shock waves and speed of sound. They weren't using a baffle, and the chrono was triggering on the shockwave of the shot, before the bullet ever crossed the sensor. Stuff like that. This one is a bit more troubling, but I'm sure it's something along those same lines. Id go back and try it again, and see if the results duplicate themselves. I'm guessing they won't.

Yeah, HI throws another wrinkle into this. I'd suggest moving your powder from the jug directly into the measure when you're about to start loading, and dumping the remainder right back in the jug just as soon as you're finished. As I said, it's highly hygroscopic, and it doesn't react well to being exposed to open, humid air for long stretches. It will change the burn characteristics, but I wouldn't expect anywhere near this much difference.

Jo191145 makes a good point here, too. Check the bore and see that you're not leaving residue or unburned powder in the bore/action. Seen this before with cases that were reloaded with moisture still in the cases (melted snow from shooting in the winter) using a stiff charge of 296. Dull report, minimal recoil so it was spotted immediately, but it left a bullet stuck in the bore just ahead of the forcing cone, and the rest of the barrel behind it literally packed with scorched (but unburnt) powder. Check it out if it happens again, and that should clear up a bunch.