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9" Barrel shootout 5.56mm vs. 6.8SPC vs. 7.62x39 UPDATE

39K views 68 replies 25 participants last post by  Col Maddog 
#1 ·
9" Barrel shootout 5.56mm vs. 6.8SPC vs. 7.62x39 UPDATE

I thought this was going to be fun to shoot all these different calibers and ammo through 9” barrels but I was wrong. I went out on the MLK holiday when the range opened hoping to get done before the crowd showed up. Turns out most of the crowd had the same idea. What should have taken 3 hours tops took 6 due to the crowding at the range. It was also unseasonably warm and I forgot my water bottle so by the time I got home I had a killer headache. I blame part of it on firing more than 1K rounds of 3 different calibers through 9” barrels for 6 hours.

On to the data. I tried to be as “scientific” as I could be. I will admit that I am not a benchrest shooter. I was using a Burris 2x7 for all the accuracy testing. I don’t have a fancy rest or anything, I mounted my Bobro Bi-pod on the front of the gun and used a small shooting bag under the back to try and be as stable as possible.

Overall, I was fairly impressed with the results, especially the 85gr 6.8SPC. It was surprisingly accurate and carried itself at a good velocity.

Of note, I fired for function checks both before and after all the official testing. Everything ran fine, the brass cased surplus 7.62x39 had a weaker ejection than any of the other 7.62x39. I think that is probably due to the brass expanding after firing. The gun may need a slightly larger gas dial as I had to shoot at Setting 4 when firing the Egyptian and Dubai stuff. All of the rest of the stuff ran fine.

I had feeding issues related to the 7.62x39 magazines when I loaded more than 20 rounds. When I loaded 19 rounds I experienced no problems but as soon as I loaded more than 20 the bolt failed to pick up the next round due to the spring not being strong enough. I experienced this on both of the C-products mags that I have.

Here is the Data, all groups were at 100yards and velocity was measured 15 feet from the muzzle. I also recorded groups at 50y using various 1x red dot optics and they were all slightly better than the 100y stuff. As the sun got brighter I experienced some red dot washout so the 50y groups weren't as good as they could have been but they were all respectable in my book.



I am sure that I am leaving something out right now but it is late. Please ask any questions that you can think of.


I shot 4 different loads in 5.56, 2 factory and 2 reloads. The reloads were a light plinking load and a load made to mimic M193.

5.56mm Velocity at 15' Energy(Ft.-Lbs.) Group Size in Inches
Smallest Largest Average
55gr Reduced Reload 2285 Avg ES=146/SD=37 637 2.70 4.00 3.35
Remington UMC 55gr 2416 Avg ES=108/SD=32 713 0.66 2.46 1.56
M193 Reloads 55gr 2475 Avg ES=68/SD=21 748 0.99 2.42 1.71
M193 Factory 2488 Avg ES=81/SD=31 756 1.57 2.27 1.92



6.8 SPC Velocity at 15' Energy(Ft.-Lbs.) Group Size in Inches
Smallest Largest Average
Hornady TAP 110gr 2150 Avg ES=37/SD=16 1129 0.69 1.95 1.32
SSA 90gr TNT 2510 Avg ES=73/SD=29 1259 1.22 1.24 1.23
SSA 85gr Barnes TSX 2528 Avg ES=76/SD=29 1206 0.56 1.60 1.08





7.62x39 Velocity at 15' Energy(Ft.-Lbs.) Group Size in Inches
Smallest Largest Average
Barnaul 123gr 2015 Avg ES=80/SD=25 1,109 2.65 4.55 3.60
Brown Bear 123gr 2073 Avg ES=56/SD=17 1,173 2.43 3.90 3.17
Dubai Brass Cased 124gr 2033 Avg ES=59/SD=18 1,129 3.60 5.35 4.48
Egyptian Brass Cased 124gr 2043 Avg ES=54/SD=16 1,140 2.03 3.70 2.87
Winchester White Box 123gr 1904 Avg ES=49/SD=16 990 2.30 4.22 3.26
 
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#2 ·
Re: 9" Barrel shootout 5.56mm vs. 6.8SPC vs. 7.62x39

nice post.

I am thinking we should just stick to 7.62x39 for CQB with SBR's, the numbers confirm it!

If you don't think so let me say this:
6.8 = $1.20 per trigger pulll
7.62x39 = $0.20 per trigger pull.
 
#4 ·
Re: 9" Barrel shootout 5.56mm vs. 6.8SPC vs. 7.62x39

nice post.

I am thinking we should just stick to 7.62x39 for CQB with SBR's, the numbers confirm it!

If you don't think so let me say this:
6.8 = =0000092136.20 per trigger pulll
7.62x39 =
nice post.

I am thinking we should just stick to 7.62x39 for CQB with SBR's, the numbers confirm it!

If you don't think so let me say this:
6.8 = $1.20 per trigger pulll
7.62x39 = $0.20 per trigger pull.
.20 per trigger pull.
As long as the mags work reliably. I guess I need to upgrade the springs in these 2 and see of that works. That or just down load them (but I havt to have such a long mag for only 20 rounds).
 
#5 ·
Re: 9" Barrel shootout 5.56mm vs. 6.8SPC vs. 7.62x39

What are people getting for 7.62x39 velocities out of different length barrels besides the 9" length in the above post? I'm trying to find a good compromise between barrel length and velocity.
 
#9 ·
Re: 9" Barrel shootout 5.56mm vs. 6.8SPC vs. 7.62x39

What are people getting for 7.62x39 velocities out of different length barrels besides the 9" length in the above post? I'm trying to find a good compromise between barrel length and velocity.
I may try to do the same type test for 16" barrels or even the longer 18" barrels.

I think that with a decent reload accuracy could improve from the 7.62x39 and get it closer to 6.8 performance. Yes, price not withstanding, I think the 6.8 is the better bet. This may be a good reason to have both though. If you want to train on the cheap with your XCR get the 7.62x39 kit. The felt recoil was very similar between 6.8 and x39.
 
#7 ·
Re: 9" Barrel shootout 5.56mm vs. 6.8SPC vs. 7.62x39

You can get the 90 g TNT loading by SSA now for .80 cents shipped, but yea, 6.8 will need a cheaper factory option or it will simply stay an expensive HD and hunting round.

Thanks for posting this
 
#11 ·
Re: 9" Barrel shootout 5.56mm vs. 6.8SPC vs. 7.62x39

Anyone know the required minimum FPS to get 55gr. FMJ .223 to frangment? What about 85-90 gr. 6.8?

I just wish the 7.62x39 mags worked....if they did....I'd get a 7.62 conversion, but since they don't....I'll likely get the 6.8 and stay away from heavier than 90 gr. bullets (and have to start rolling my own for this round).

Sean
 
#12 ·
Re: 9" Barrel shootout 5.56mm vs. 6.8SPC vs. 7.62x39

Anyone know the required minimum FPS to get 55gr. FMJ .223 to frangment? What about 85-90 gr. 6.8?

I just wish the 7.62x39 mags worked....if they did....I'd get a 7.62 conversion, but since they don't....I'll likely get the 6.8 and stay away from heavier than 90 gr. bullets (and have to start rolling my own for this round).

Sean
I purchased a metric butt load of 115gr 6.8 bullets when Midway had them on sale. I need to start loading them and see what I can get them to do.

I think that the mags work just fine with an upgraded spring, I just wanted to try them out without any mods.
 
#15 ·
Re: 9" Barrel shootout 5.56mm vs. 6.8SPC vs. 7.62x39

What's up with the velocity on the 6.8 Hornady TAP round? ???
Short bbl....heavy bullet. My limited understanding is that if you roll your own, you can get them a lot hotter....the whole SPC vs. SPCII chamber debate has meant a lot of downloading of factory ammo to avoid overpressure issues in the tight chamber guns.

Sean
 
#17 ·
Re: 9" Barrel shootout 5.56mm vs. 6.8SPC vs. 7.62x39

Is standard FPS testing done with the chronograph at 15 feet? Why?....too much muzzle blast closer in that skews results?

Sorry, I don't know anything about chronos or the standardization.....just curious.

Sean
 
#19 ·
Re: 9" Barrel shootout 5.56mm vs. 6.8SPC vs. 7.62x39

Is standard FPS testing done with the chronograph at 15 feet? Why?....too much muzzle blast closer in that skews results?

Sorry, I don't know anything about chronos or the standardization.....just curious.

Sean
I honestly just have seen others post that amount so that is what I have been using. That is about the length of cord that I have that goes from the chrony to the digital readout that sits on the bench so that is where I put it.

I wouldn't be too concerned about the velocity loss, remember that same TAP round out of an 7" barrel was 2044fps, so it looks like it is averaging about 50fps per inch at that point. Remember, from 11 to 16" there was less than 100fps difference. http://xcrforum.com/index.php/topic,5007.msg73191.html#msg73191

I really need to load my own and see what I can come up with. That and I will get some 85gr TSX combat loads and see how they do.
 
#18 ·
Re: 9" Barrel shootout 5.56mm vs. 6.8SPC vs. 7.62x39

I wish I would get some of the malfuncs that peops complain with the 7.62 x 39. Just to see what they are talking about.

I only have the 7.62x39 with 16 inch bbl and 6 30rd C-mags. I have yet to have any malfuncs whether they are loaded with 20 rds or 30 rds.

Could be the generation of the Cmag. I bought mine in November, that's the gen 4s right?
 
#20 ·
Re: 9" Barrel shootout 5.56mm vs. 6.8SPC vs. 7.62x39

Is that TAP round a vmax or a BTHP?

While it makes more sense for SBR's, I really question the trend for every more lighter bullets for 6.8. 115 g is already light for caliber. 85 g extremely light - what would be the equivalent 22 bullet, 30g's?? Seems like you are getting so low on section density and bullet momentum you might have terminal ballistic performance problems. Below 100 g why not reach for a heavy 5.56? Anyone see any FBI protocol tests comparing these very light 6.8 vs heavy 5.56?
 
#21 ·
Re: 9" Barrel shootout 5.56mm vs. 6.8SPC vs. 7.62x39

The 6.8 looks better all around, while 7.62x39 looks the best form a value point of view.

AZ, if yo get a chance can you try 77/75 grain 5.56 tips and let us know the result.
 
#22 ·
Re: 9" Barrel shootout 5.56mm vs. 6.8SPC vs. 7.62x39

Crap, now I just need to see the same tests for .308 and 6.5. If I can find a system for a quick change barrel in 6.5 I might eventually try it. Thanks to some of you guys, I've gotten myself another FAL, a 16" Para Tactical with short gas system. I found a guy through falfile who does a QC barrel set up for the FAL. He isn't doing them right noe, but he said to give him a call this summer-he's retiring from a teaching job and plans to go back to gunsmithing full time. That'll give me time to get the SBR paperwork done, so I'll have a 16" and 11" .308. Just need to find an economical way to change barrel length on the 6.5 and maybe I can test this summer-I already have a 16" 6.5...
If the 6.5 performs as I hope in 9" or 11" barrels, all I need is for Robarm to get it done!!! Alex, if you read this, I'll still be glad to beta test the 6.5!! I went with the FAL because I figure you can never go wrong with .308, and with the QC conversion I might get it set up for .338 federal, too-even at its best I don't think 6.5 will ever appraoch THAT capability...
 
#23 ·
Re: 9" Barrel shootout 5.56mm vs. 6.8SPC vs. 7.62x39

That would be interesting, can you post his details?
 
#30 ·
Re: 9" Barrel shootout 5.56mm vs. 6.8SPC vs. 7.62x39

Aziator....


Just looked at the thread you linked to. From what's there it *seems* (and correct me if I'm wrong) that the 7" bbl was running around 1900 fps on average. The 11" bbl was running about 2300 fps on average. Is my math really off or is it more like 100 fps per inch of bbl lost going by those numbers (at least for 115 gr. bullets)?

Yes, I agree that there's only around 17 fps lost per inch from 16" down to 11" (in the 115 gr. bullets)....but once you get shorter than 11" that seems like a pretty significant drop.

Thanks for any clarification.

Sean
 
#35 ·
Re: 9" Barrel shootout 5.56mm vs. 6.8SPC vs. 7.62x39

Aziator....


Just looked at the thread you linked to. From what's there it *seems* (and correct me if I'm wrong) that the 7" bbl was running around 1900 fps on average. The 11" bbl was running about 2300 fps on average. Is my math really off or is it more like 100 fps per inch of bbl lost going by those numbers (at least for 115 gr. bullets)?

Yes, I agree that there's only around 17 fps lost per inch from 16" down to 11" (in the 115 gr. bullets)....but once you get shorter than 11" that seems like a pretty significant drop.

Thanks for any clarification.

Sean
The problem with the 7" test is that I didn't shoot the Hornady through anything other than the 7". I should have shot it through the 11 and 16 to get some solid numbers for it on that day at that temp. I also think that the SSA stuff I was shooting was a little weaker than average. It was older stuff that I had from a couple years ago.

Bottom line, I need to roll some 6.8 of my own and do a comparison between all the barrel lengths. I am pretty sure I can push it a little further than they are doing. If I get some time this week I will try to load some up.
 
#33 ·
#32 ·
Re: 9" Barrel shootout 5.56mm vs. 6.8SPC vs. 7.62x39

Sean, for velocity to drop off like that means you wouldn't be getting as complete a powder burn as you would with the longer barrel. Less burn, less pressure. Somewhere in between there is the sweet spot. With 6.8 that makes sense since they designed with short barrels in mind, IIRC.
 
#34 ·
Re: 9" Barrel shootout 5.56mm vs. 6.8SPC vs. 7.62x39

Sean, for velocity to drop off like that means you wouldn't be getting as complete a powder burn as you would with the longer barrel. Less burn, less pressure. Somewhere in between there is the sweet spot. With 6.8 that makes sense since they designed with short barrels in mind, IIRC.
Ya, 6.8 was designed with short bbls in mind....but I don't think they really went below 10". I was pretty set on a 6.8 conversion for my XCR mini.....but with the cost (and since I already have 6.8 mags out the ying yang), I may just go ahead and get a 7.62x39 if I can get the mags to work.

Sean
 
#36 ·
Re: 9" Barrel shootout 5.56mm vs. 6.8SPC vs. 7.62x39

Excellent work, Aziator! I feel for you about the bad headache, though.
When ever I spend 6+ hours shooting, I usually come back with a headache, too!
It's really interesting to see how each calibre reacts to a shortened barrel - thanks for that info.
I'm sure it'll help many people make a wise choice when picking a short barrel.
Were the SSA loads thier commercial or tactical loads?

Oh, and I'll be working up some 90 grain Speer TNT loads for testing in my 7.5" 6.8SPC soon. I'll be using Reloader 7 and LilGun for my loads.
I think I'm 2-3 weeks away from testing, though. My newest brass order has been delayed - just waiting on that.
 
#37 ·
Re: 9" Barrel shootout 5.56mm vs. 6.8SPC vs. 7.62x39

I wonder if some creative loading could be accomplished, using faster powder to reach optimum pressure before the bullet exited the barrel. As long as pressure doesn't excede safe level it might allow higher velocity out of an SBR. Thy wouldn't be safe in regular length barrels but could perform nicely in the stubbies.
 
#38 ·
Re: 9" Barrel shootout 5.56mm vs. 6.8SPC vs. 7.62x39

Yes, it's the extra power AK springs. And note the price per spring difference if you buy them in packs of 10.

So Aziator, the two 7.62x39 mags you used, did that include the 'updated' mag you got from Alex?
 
#40 ·
Re: 9" Barrel shootout 5.56mm vs. 6.8SPC vs. 7.62x39

Yes, it's the extra power AK springs. And note the price per spring difference if you buy them in packs of 10.

So Aziator, the two 7.62x39 mags you used, did that include the 'updated' mag you got from Alex?
Ya, I noticed that.....$5.50 a mag ain't too bad if you buy the 10 pack. Thanks for the info. Gives me something to really think about. What about the funky curve of the CProducts X39 mags? Do they fit in a mag pouch okay? I'm thinking at most you could only fit one and even then, I'd guess the curve would mean putting two next to one another might be an issue.

Sean
 
#42 ·
Re: 9" Barrel shootout 5.56mm vs. 6.8SPC vs. 7.62x39

They fit a little better than the AK because they are slimmer and they don't have the locking tab that is present on the backside of the AK mag. Thats if you put the mag in with the opening down, which is what I do.

Placing them in adjacent pockets is no problem either, just make sure that the curve goes the same way ;)
 
#43 ·
Re: 9" Barrel shootout 5.56mm vs. 6.8SPC vs. 7.62x39

I don't own an AK (believe it or not ;) ) so I all I currently have is mag pouches for SR25 type 308 ARs or 5.56/6.8 ARs. I hate to have to swap out all that gear to change, but it's good to know that Lizardman says they may work with the pouches/vests/chest rigs I already have.

Thanks,
Sean
 
#44 ·
Re: 9" Barrel shootout 5.56mm vs. 6.8SPC vs. 7.62x39

I'm sure it depends on the pouch, but they won't fit in mine for 5.56. I know some companies will make them that are intended to fit both. They might fit in the .308 pouches though if they're tall enough. I guess the only way to know for sure is to try it.
 
#46 ·
Re: 9" Barrel shootout 5.56mm vs. 6.8SPC vs. 7.62x39

I'm sure it depends on the pouch, but they won't fit in mine for 5.56. I know some companies will make them that are intended to fit both. They might fit in the .308 pouches though if they're tall enough. I guess the only way to know for sure is to try it.
You're probably right. Guess I'll need to buy one just to check before getting a conversion.....but that's a ways off anyhow.....I'm still trying to pay off all my credit card debt before the zero percent runs out. ;)

Sean
 
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