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Posted: 5/12/2019 11:58:37 PM EDT
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Short answer: There was some benefit to the faster powder vs the slower powder as the barrel got shorter, but not by a great deal.
The purpose of todays test was to observe the effects of fast vs slow powders as BBL length changes. It's been said that slow powders will not do as well in a shorter barrel. And others have said that is in error, and that the powder is essentially all burned up in the first few inches of BBL, beyond which it does not matter. There's basis for both claims. Now that AR's with barrels as short as 12, 10.5, 8, and even 7" have grown much more prolific in pistol configurations, the question has never been of greater relevance. To test this, 3 guns of varying BBL length were run: using a slow powder load, and a fast-powder load. Test objectives: 1) Is there a measurable difference in a fast gunpowder vs a slow gun-powder in the delta muzzle velocity going from a 20" to a 16" to a 10.5" BBL length. i.e., is the fast gunpowder more optimal in a shorter barrel length in terms of velocity behavior? 2) Is there a measurable different in fast vs slow gun powder muzzle blast change, when barrel gets shorter. i.e. if running a suppressor (or muzzle break), will the fast gunpowder have relatively lower blast as barrel length gets shorter, vs what a slower powder is doing? 3) Note any performance or other observed difference, such as recoil, cycling reliability, etc, based on gas system and BBL length. A friend joined me today and I had access to an 10.5" AR pistol for this testing. Guns: 20": Rifle gas system, 1/8 malonited stainless steel. 16": Mid-length gas system, 1/9 twist chrome lined. 10.5" pistol: carbine gas system, 1/7 malonited steel. Ammo: Hornady 55 FMJ ball; loaded long (2.25"). One powder from the fastest end of what is appropriate for this bullet, and one powder from about the slowest. LT-32. This is a fast powder, similar to 4198. With this powder, 55 gr is considered the heaviest .223 bullet to run. It has all the latest copper fouling and temperature insensitivity technology in it; and I actually quite like this power. Ran it at 23.5 gr, which fills the casing up into the shoulder. Loaded above SAMMI 223 spec, and into SAAMI 5.56 spec pressure (with 0.3 gr to spare, so not crazy hot). MP530 (which is CFE223). This is a slow powder, and my lot is actually on the slow end of even that, so my charge is not necessarily appropriate for canister grade CFE 223. Ran 27.9 gr. Test Methodology Velocity: Ran a LabRadar for velocity tracking, and fired either at paper or a gong, at 200 yards distance. Muzzle blast: Ran a decible meter app on my smartphone, and placed at a reference distance from the guns. Higher decible recordings, mean higher muzzle blast. Alas, Unfortunately, my muzzle blast test failed. The smart-phone microphone would top out around 85 decibles, so all the shots basically recorded at the same noise level. Placing the phone further away would likely correct this, but that wasn't feasible as others were shooting at the range as well. This could be done on an empty range and just placing the phone farther away, but from my testing, 20' wasn't nearly far enough. Results 55 FMJ LT-32 ammo (Fast powder): 
55 FMJ CFE 223 ammo (Slow powder): 
Other observations. -In the 20" rifle, even though the LT-32 ammo was 100 fps faster, it actually had notably milder recoil compared to the CFE223 ammo (see Assessment for why). -Fired casings from the 20" rifle did not go far. -Fired casing ejection and felt recoil was indistinguishable between the LT-32 and CFE223, in the 16" and 10.5" guns. -For some reason, it seemed like the fired casings from LT-32 ammo were cooler, than the fired casings from the CFE223 ammo. Odd. This was not measured or quantified and may be an incorrect observation, so for now we'll call that conjecture. Assessment -The faster burning LT-32 did appear to outshine the slower burning CFE223 powder as barrel length got shorter, but only by a little. The faster LT 32 had a 1% velocity loss going from 20" to 16" (compared to 2% velocity loss the CFE experienced). And going from 20" all the way to 10.5" resulted in a 15" velocity loss for the LT-32, while the CFE223 had a 17% velocity loss. One must have care making too many conclusions since these differences are within standard deviation (or close to it); but are repeated throughout the testing, prior testing, and replicates. -The fast pressure burn curve of the LT-32 was indistinguishable in a 10.5" carbine or 16" middy gas system compared to the CFE powder, but in a 20" rifle length gas system it was notably milder. No doubt due to a much lower port pressure than the slower CFE223 powder has in the 20". -The standard deviations weren't that impressive for either powder and load, but it is curious that the LT-32 had the same standard deviation in every barrel (that's not a typo). -The LT-32 had a more attractive ft/s loss with decreasing BBL length, than the CFE223 powder did. -The LT-32 had 1 failure to feed in the 20", which was similar to a short-stroke. It actually appeared to be magazine related (20 round PMag), where the ass of the next round did not fully lift, and was slightly depressed, allowing the bolt to ride over it. While it is convenient to blame the magazine, I actually only partially do so. Magazines are by-design supposed to be jostled a little bit by the recoil, to aid in proper cycling. And since the 20" had notably mild recoil, one wonders if the round would have feed up and flush against the lips had it been more jostled from heavier recoil. Indeed, the same mag had no cycling issues in the heavier recoiling 16" and 10.5" BBL guns. Or it may have just been a straight up short-stroke. -Going from a 20" to a 10.5" BBL, the faster LT-32 had a 458 ft/s velocity loss, vs CFE 223 with 505 ft/s loss. Conclusion 1) There was measurable benefit to the faster powder vs the slower powder, as BBL length gets short. Though not by much. 2) My decible test method didn't work. I suspect the answer is yes, if nothing else as evidenced by the much milder port-pressure observed in the rifle length gas system with the faster powder (despite still having higher muzzle velocity). 3) In a 20" rifle gas system, the fast LT-32 powder in this case appears to be at the edge of suitable. I'd run it, but maybe not as primary for anything serious. For the more common 16" (and growing more common 10.5"; i.e. not rifle length gas systems), I'd run it all day; preferentially (if pushing 55 FMJ). |
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Good post but it looks like you may have a slow 20" barrel, a fast 16" barrel, or a combination. When the same barrel is cut off from 20 to 16 inches you usually end up seeing a loss of about 100fps. But ignoring that your results suggest it's not that big a thing to worry about.
Rem 700 barrel cut off to reduce variables. |
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Quoted:
Good post but it looks like you may have a slow 20" barrel, a fast 16" barrel, or a combination. When the same barrel is cut off from 20 to 16 inches you usually end up seeing a loss of about 100fps. But ignoring that your results suggest it's not that big a thing to worry about. Rem 700 barrel cut off to reduce variables. |
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IMHO........
If you’re using MP530 for the test, call it MP530, particularly if you admit that the lot of MP530 you are using isn’t close to CFE223. Which chart are you using to describe them as fast/slow powders? I’d be interested in seeing your results with other common powders. |
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Quoted: Good eye. i thought so as well. But, that's what the data said. With multiple replicates. Maybe my 20" is slowing down, it has a few rounds on it. Quite possible a new 20" chrome lined BBL would be faster, though the good news is it was an equal-conditions test between two loads, so the results and conclusion aren't really affected, at least. |
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