300 BLK Velocity by the inch

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MMH
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300 BLK Velocity by the inch

Post by MMH » Sun Dec 06, 2020 4:22 pm

I am in the process of getting my first suppressor. It will be a multiuse can (Griffin Bushwhacker 46), but my initial focus will be on suppressing my 300 Blackout. I currently have a Sig MCX Gen2 rifle - 16" barrel. To me a 300 BLK does not need anything close to a 16" barrel. So I am going to do a velocity by the inch analysis - I will chonograph loads and cut the barrel down 1" at a time & then shoot the I will be concerned about supersonic & subsonic perfomance. The supersonic will be predominantly 110 & 125 grain bullets and the subsonic 190 grain (assuming I'll be able to get some Hornady Sub-X bullets in the near future.

I will chronograph (labradar) supersonic loads for the 110 & 125 grain using H110. My maximum load is 20.2 gr. of H110 under a 110 gr. GMX bullet & am getting a velocity of 2460 FPS. I have not developed my subsonic loads but will use a 225 gr. ELD-M & 1680 powder. Target velocity will be 1050. After getting velocity w/ a 16" barrel, I am going to cut down the barrel by 1" & get updated velocity data. The 110 gr. loads will stay constant, but I will adjust the subsonic loads as necessary to maintain 1050 fps. For Supersonic, my objective is to see how the velocity drops off with barrel length. With subsonic, the objective will be to maintain reliable function. I will not want to give up too much supersonic performance and expect that I will end up with a barrel length of somewhere between 10" to 12". The shortened barrel would be mounted in a pistol lower so that I do not create a SBR in the process.

After the testing is completed, I will permanently attach a 'muzzle device' to get the barrel length out to 16" and minimum overall weapon longer than 26". At this point I can legally mount it again on a rifle lower. Purpose of muzzle device is to avoid the hassle of having an SBR rifle. The muzzle device will be little more than a thin (.45 OD, .375 ID) tube with many holes drilled in it. The multi-use suppressor will slip over the muzzle device and effectively attach at the end of the barrel and not at the end of the muzzle device. This way the overall length of the rifle with the attached suppressor will be kept short. With a MCX (which does not have a buffer tube, I think that I will a suppressed rifle that has a 11" barrel & 28" length w/ folded stock.

Comments on my plan?

johndoe3
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Re: 300 BLK Velocity by the inch

Post by johndoe3 » Mon Dec 07, 2020 1:21 pm

I would prefer that you also hold the subsonic test to a constant to see the dropoff of speed for subsonics too as the barrel is shortened. You can always change the cartridge powder charge for the subsonic load at the shortest barrel length to achieve 1050 fps to compare it to the 16" barrel. In between, there should be a straightline powder charge change for subsonics load from longest barrel to shortest. This change would make your overall test faster and more meaningful.
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MMH
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Re: 300 BLK Velocity by the inch

Post by MMH » Mon Dec 07, 2020 1:44 pm

johndoe3 wrote:
Mon Dec 07, 2020 1:21 pm
I would prefer that you also hold the subsonic test to a constant to see the dropoff of speed for subsonics too as the barrel is shortened. You can always change the cartridge powder charge for the subsonic load at the shortest barrel length to achieve 1050 fps to compare it to the 16" barrel. In between, there should be a straightline powder charge change for subsonics load from longest barrel to shortest. This change would make your overall test faster and more meaningful.
Good point. However, if I do not get reliable function (w/ the 190 gr. bullet) then I will increase charge weight to get back to 1050 fps. I have a 16" barrel & my gas port is approximately 6.25" from the bolt face. I'm thinking that even with the barrel cut down to 11" or 12" there should be enough dwell time to cycle. Hopefully I will see what happens as I go down from 16" to 15", etc. and get a feel for it. I would hate to cut the barrel down and then not be able for have proper functioning of the gun. In addition, I would still have the ability to go to a heavier bullet (225 or even 250) if the 190 does not cycle properly.

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Capt. Link.
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Re: 300 BLK Velocity by the inch

Post by Capt. Link. » Mon Jan 04, 2021 3:43 pm

I'm not sure if the experiment is truly needed today. The large numbers of powders and bullet weights should allow almost any velocity in almost any length barrel.
I can say that many moons ago SSK claimed that the 300 whisper was optimized with a eight inch barrel. The powders of the day were limited and a longer barrel made the round supersonic. The subsonic projectile power and accuracy was king! My whisper using 240gr SMK bullets delivered 1/4 MOA and a bunch of power @ 900fps is still a great load. I'm surprised the weight of bullets are lighter in the blackout.....why, subsonic performance depends on weight and penetration.
Don't get me wrong the information will be valuable. You might gather the muzzle pressures along with velocity. A point of impact study along with sound pressure would make this epic.
Best-CL
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noname
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Re: 300 BLK Velocity by the inch

Post by noname » Thu Jan 21, 2021 11:02 pm

300 BLK is a SAMMI round where-as Whisper is not, therefore you can lookup SAMMI loading information for rifles and find a lot of useful information. My primary reloading manual is Lyman 50th edition Reloading Manual.

I have an 8 inch 300 BLK AR suppressed and cannot tell the difference in it and a 16 inch barrel suppressed. You really need information from a reliable source that has an expensive db meter measured beside the muzzle usually 3-4 ft away (to the side) if you are going to nit-pick noise levels from a rifle. Powder and bullet makes a difference too. The ones available for an I phone are useless for comparing.

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