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Posted: 8/26/2013 4:59:25 PM EDT
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Not looking for answers from web shooters here. Just authorities. And I certainly appreciate links to authorities threads on the subject. Looked through too many threads that are muddled with baseless opinions.
BTW, Thanks to Molon for all his efforts! I have a 10.5" 1/7" noveske barrel chambered in 5.56 that I have been avoiding for quite some time. Id like to work up some loads for it but for now what im wondering I guess to start is there anyone out there (factory or home) working up heavy loads like the 77gr that can take advantage of this fast twist rate and still keep the velocities up for good frag in this super short barrel? If home loads...recipe or Is there a powder I should be looking at for this? What is the highest velocity a guy could ever expect to get from a load like this if he pushed the pressures. Thanks! TS |
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Velocity is dependent on pressure. Just because you have a short barrel doesn't mean you can run more pressure. 5.56 is already about as high a pressure round as it can be.
You might be able to run a faster powder and get some extra fps but its not gonna be worth it. 77gr smk ain't what id be doing. The 50gr or 70gr TSX is where it's at. The 70gr expands reliably as low as 1800fps. The 50gr at 2300fps but it can be launched a bit faster then the 70gr, obviously. |
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Without a Chrony estimating your velocities is basically a WAG.
Here is my best guess. Ramshot data for the 5.56 77gr Matchking with TAC is 2902fps out of a 24 inch barrel. Plugging this data into Quickload, a ballistic calculator, then reducing the barrel length from 24 inches to 10.5 inches we get an estimated velocity of 2380fps, a loss of 522fps. Next guess would be to go here and check out this test. They tested velocity on progressively shorter barrels in the 223 Rem., from 22 inch down to 10 inch. My rough math shows and average velocity drop of about 45fps/inch of barrel. That means using 24 inch barreled data you would loose approximately 600fps in 10.5 inch barrel bringing load data velocities down to 2300fps. http://www.accuratereloading.com/223sb.html Ramshot and Accurate had a footnote in their 223/5.56 load data that would help explain and calculate loss in velocity due to barrel length. They removed it because people were to stupid to know that it was for a velocity calculation NOT a powder reduction calculation. |
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I am sure this is of no value to you but FWIW... I did back to firings with the same loads in my 16" AR and my 7.5" AR I lost an average of 617fps with 55gr and 62gr FMJBTs. 55s were 2775 and 2120(655 dif - 77fps/inch), 62s were 2640 and 2060(580 dif - 68fps/inch).
The 62s were 22.8gr of AA2230 The 55s were 23.5gr of AA2230 |
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I tested some Hornady 75gr OTM reloads last week in my LMT 10.5".
75gr Hornady BTHP Black Hills Match Brass 1x CCI 450 TAC - 25.2gr OAL 2.260" Light crimp with a Lee factory crimp. 75F no humidity I followed Ramshot's published guide for 62,000PSI+ barrels (5.56 chambers) and they list the max load of TAC for that bullet at 25.8gr. Right now I have plenty of velocity and I'm curious how much more I can get before I start to see pressure signs and accuracy decreases. Velocities: 2487,2482,2478,2442,2482 Average: 2474 FPS STDEV: 18 FPS Hope that helps! ETA: Photos of the primers / case heads: 
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Quoted:
I have a 10.5" 1/7" noveske barrel chambered in 5.56 that I have been avoiding for quite some time. Id like to work up some loads for it but for now what im wondering I guess to start is there anyone out there (factory or home) working up heavy loads like the 77gr that can take advantage of this fast twist rate and still keep the velocities up for good frag in this super short barrel? Not sure why you're "avoiding this". My home defense gun is a 10.5 will 77 gr SMKs. It's the perfect combo. 23.4 grains of XBR is a Mk 262 clone that is OVER published max, so work up to this carefully. It frags like a monster. Don't have my chrono book handy at this moment. 22.8 grains of H322 is a max .223 load that when combined with Wolf SRMs is a DO ALL match, defense, Long range, and SBR CQB round. The same goes for the XBR load above... Just that the H322 is published max. |
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I'm curious how much more I can get before I start to see pressure signs and accuracy decreases. You may never see a decrease in accuracy working up to published max. There is no hard and fast rule that top velocities and max pressures will cause a decrease in accuracy. You may even find your accuracy improving as you work higher. The only way for you to find out "how much more you can get" is to work the load up for yourself and see. Max pressure for the 5.56 is 62K, the pressure signs you seek will not magically appear when you have crossed the 62K threshold. |
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Yeah.. you want to shoot for a reasonable velocity. A buddy of mine thoughly works and XBR load that exactly cloned Mk 262 like I mentioned above. My charge is backed down a little to give me some margin for error.
The beauty of the 77 SMKs is you don't need to scortch the hell out of them to get tremendous frag and penetration. 
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Quoted:
Not sure why you're "avoiding this". My home defense gun is a 10.5 will 77 gr SMKs. It's the perfect combo. 23.4 grains of XBR is a Mk 262 clone that is OVER published max, so work up to this carefully. It frags like a monster. Don't have my chrono book handy at this moment. Just one of those priority things. It was under-gassing and got back burnered. I have it working well now with M193/855 and I will have time to work on this one whilst watching football in the next couple weeks to work up something more befitting. Thanks so much to all the guys here! All of these post give me some great places to start. I will reciprocate your help with posting my experience with these loads. I've got a bunch of knotts and maybe I'll go as far as making up a couple blocks to blow up after chrono. |
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