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Follow up on SBR ballistics - 16" vs 10.5" - not nearly as much as I expected

Well clearly, I've been bitten by the SBR bug again. I don't know if it's the cool factor, or some practical potential, but it looks like whether I want to be or not, I'm back into the SBR.

I ordered an AR Stoner 10.5" barrel - that's about the only thing I could find! Then later I managed to find a Seekins Precision 10.5" so it's on the way too, and I'm thinkin' it's a good thing it is - more about that comin' up.

So I got the AR Stoner 10.5 set up, took off to the range, attached the MagnetoSpeed Chrono and got busy.



I as actually surprised - pleasantly! I was expecting a pretty big drop in velocity, but with the 77 gr ammo I was using it looked like this:

Barrel lenVelocityEnergyMomentum% diff vel% diff enrg% diff mom
VooDoo 16", 1:72680122729.48---
VooDoo 11.5", 1:72477104727.258.2%17.1%8.2%
VooDoo 10.5", 1:72513108027.66.6%13.6%6.6%

One thing that bothers me a bit is that the 10.5" velocity is actually higher than the 11.5" velocity. I rechecked my notes, and those are the numbers I recorder. I'll probably reshoot those to be sure.

The percentages use the 16" barrel as the reference and the percents represent the velocity loss from the 16" barrel. The 10.5" only gives up 8.2%! That's what pleasantly surprised me. However, all is not good.

The accuracy from the 10.5" was awful. I didn't take pics but 3-4" groups were common. And it didn't seem to be a barrel drift issue; the 10.5" was NOT a light weight barrel. The accuracy is gonna be really hard to explain seeing there was only an 8% difference in velocity from the 16" to the 10.5" and both have a 1:7 twist.

My 12.5" barrel had an average velocity of 2595 fps and it had great accuracy. So from 12.5" and a velocity of 2595 to a 10.5" with a velocity of 2513 fps, that's only 82 fps difference or 3.3% difference.

I was shooting with a suppressor, and it is possible that the bore of the 10.5" was a bit off center with respect to the threads. I checked that pretty carefully though and did not see a problem there.

I will continue to "investigate".

However the great news is the 10.5" just doesn't give up that much velocity compared to the 16" barrel. Of course it could be that the 16" barrel is shooting a bit slow. I'll compare it to another 16" just to be sure. And I am eager to see how the 10.5" Seekins Precision barrel compares the the AR Stoner - that should happen next Wednesday.

I guess the question remains, why a SBR that's only 5.5" shorter? I have a specific answer for that: I don't know!
 

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I guess the question remains, why a SBR that's only 5.5" shorter? I have a specific answer for that: I don't know!
The reason that SBR's are becoming popular is mostly because of silencers.

10 years ago, no one gave a hoot about SBR's. Now, if you decided to own one, an 8" or a 10" silencer adds a lot of length to the rifle.

With an 8" 9". 10.5" or an 11.5", its a much more compact unit when the silencer is attached and lots of people, hunters in particular have figured that out. It's so much easier to handle an SBR that many people are will to pay the 200 tax to make it happen.

As a matter of fact, it isn't just the AR's or the AK's that are getting SBRr'ed, its bolt actions too. I've been cutting a lot of rifles off just to SBR them, some of them right at the forearm just to save on the length when a silencer is attached. I cut 11 of them this week.

Even .22's are getting cut. A popular conversion is the Ruger Take Down .22. Backpackers and Hunters are having them cut to 11" (right to the forearm) and mounting a 6" .22 can to them to make a nice, handy, easy to pack rifle.

Yes, in many calibers it loses speed when the barrel is shortened. Some calibers lose more than others. The .300 Blackout and the 7.62x 39 are popular SBR's because they give up little in an SBR as compared to a 5.56.

However, when shooting suppressed you want to use subsonic ammo for maximum quietness so the shorter barrels aren't usually an issue. It you want speed, you just use a different rifle. It you want quiet, you use an sbr. You just choose the right tool for the job.


Another aspect that is pushing AR's is this new age of unlawfulness in which common low classed thugs are allowed to block off major interstates. People are arming themselves against stupid, and a gun that you can put in your lap and shoot through a window makes a lot of sense.

I sold 3 of them that week specifically for that purpose but that doesn't change the fact the the BATFE is being flooded with Form 4's for SBR's.

The advantages of the SBR's are numerous and they will only continue to become more popular. The $200 tax stamp is no longer the deal killer that is was meant to be and people are just paying it and waiting.
 

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Question for you Tangle.

How are you liking that chronograph? I've though about getting one...just easier to set up.
 

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I hear ya HotGuns! My question was intended as tongue in cheek. Plus, I made a mistake which I have corrected. I said, "...that's only 3.5" shorter?..." That should have been, 5.5" shorter - 16" - 10.5" = 5.5", for some odd reason I had the 12.5" barrel on my mind???

But, yeah, the benefits of SBRs are many. For me, it's much like you said, when I add a Trek T suppressor that's 5.7" long to a 10.5" barrel, I get an overall barrel length effect of 16.2" - just about the same as an unsuppressed 16" AR. Typically though the concern is the loss of velocity by the 10.5" barrel and while there is some, it's not nearly as much as I expected it to be - only about 8%, IF the velocity I got for this load from the 16" barrel wasn't unusually slow, and I doubt it was. It was the same ammo from the same box.

As to the MagnetoSpeed, I love it. You don't have to align screens, worry about rain or shadows, and you get the velocity about 4" from the muzzle instead of 10 feet. However if you attach it to a suppressor, it pushes the muzzle to the first sensor out by the length of the suppressor - that's still a whole lot closer than the ten feet needed for screen types.

I've done a few corroborating experiments and the MagnetoSpeed appears to be quite accurate.
 

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Have you shot the 10.5" without the can? That might help determine some of the accuracy issue. I like the idea of a handy rifle that doesn't need ear protection to use.
 

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Have you shot the 10.5" without the can? That might help determine some of the accuracy issue. I like the idea of a handy rifle that doesn't need ear protection to use.
I shot it twice without the can and then the range maintenance guy came up and started talking. He's a friend so I wound up using what little time I had left gabbing instead of shooting.

I shot it with the can today after the club picnic and trade day. Only the plinking range was open, but it seemed like it was pretty well on. I'll be going back to the range tomorrow afternoon and shoot some targets. If I don't see what I want with the can I will be sure to try it without the can this time.
 
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