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I'm looking to build my first AR pistol in 300blk and am seeing that the ideal barrel length is 9" - that little is gained by stretching longer while less would let the bullet leave before all of the powder is combusted.

My question is, what happens if one were to use, say, a 5" barrel and a 4" suppressor...would this, ballistically, be nearly identical to a 9" barrel?

Please feel free to answer that question, correct any wrong assumptions I have, and/or offer any 300blk/AR Pistol advice you care to share. Thanks!
 
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Suppresors aren't rifled. Your 4" barrel will come with the decrease in velocity whether the suppressor is attached or not.

What you're more likely to have a problem with is stabilizing the bullet. Baffle strikes become an issue, so you'll need to do a bunch of testing with your loads on paper before attaching the suppressor.
 

Ura-Ki

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NO, a suppressor doesn't add to the functional barrel length! Ideally, you want to find out where the powder stops accelerating the projectile and use that to determine minimum barrel length! No idea where .300 ends up on that, so I cant say. If the goal is overall short length, consider a reflex type suppressor that extends back over the barrel with a rear blast/pressure reducing chamber! Ultimately, your goals are going to contain a bunch of trade off's, so you need to consider which are acceptable and which are not and go from there! There are also the practical limits on what's available to you, so that needs to play in to this!
 

ilikegunspdx

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I'm looking to build my first AR pistol in 300blk and am seeing that the ideal barrel length is 9" - that little is gained by stretching longer while less would let the bullet leave before all of the powder is combusted.

My question is, what happens if one were to use, say, a 5" barrel and a 4" suppressor...would this, ballistically, be nearly identical to a 9" barrel?

Please feel free to answer that question, correct any wrong assumptions I have, and/or offer any 300blk/AR Pistol advice you care to share. Thanks!
What do u mean by nothing gained over 9”? Haven’t looked at graphs of subsonic but for supersonic ammo the gains beyond 9” are big. 581CB46B-F098-41D4-AD42-C67A7E4E25B8.jpeg
 
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ilikegunspdx

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NO, a suppressor doesn't add to the functional barrel length! Ideally, you want to find out where the powder stops accelerating the projectile and use that to determine minimum barrel length! No idea where .300 ends up on that, so I cant say. If the goal is overall short length, consider a reflex type suppressor that extends back over the barrel with a rear blast/pressure reducing chamber! Ultimately, your goals are going to contain a bunch of trade off's, so you need to consider which are acceptable and which are not and go from there! There are also the practical limits on what's available to you, so that needs to play in to this!
Right! Trade-offs is the key word to my way of thinking. Gotta find best solution for an individual’s needs/intended use.
Fe Small size of weapon vs loss in velocity/energy is something to consider (ie how much loss can u live with for benefit of small size?) If subsonic is a goal, bullet choice and barrel length to keep it subsonic is also something to consider.

Also what range u intend to shoot at and amount of drop (and energy) u can live with. I know one source (wideners pasted below) used an often quoted assumed (not measured) 25 FPS loss per inch of barrel length. However if u look at the fenix data it shows an avg 44 FPS per inch for supersonic round. Anyway, using the assumed 25fps loss rate, wideners calculated a 47” bullet drop (and only 280 ft/lbs energy) at 200 yards for 190 gr subsonic round. I wonder how big drop would be if they used an assumed value of 44fps loss (u could plug it into a ballistics calculator to find out if u want). It’s Probably a heck of a lot. that doesn’t mean it’s “bad”, it just means that if u intend to shoot subsonic at any significant distance through a short barrel u may be really surprised at the amount of holdover needed and how much energy bullet has (ie it’s a trade off).
4E4D105C-CCFA-4687-A7B3-A66CA76A7056.jpeg
Also fenix ammo data for Supersonic for reference:
095206D2-9FBD-4B1E-9A47-D1E258691DA7.png
 
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ilikegunspdx

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Another thing to think about re length is if a person is going for small size for storage such as with a truck gun etc. a 16” barrel sig virtus or other “bufferless” 300 BO will be the same overall length (with stock folded, not deployed) as a 9” barrel 300 BO AR with a buffer. Both are roughly 26” (sig with stock folded, not deployed). That may be a factor for one person (fe someone where small stowed/transport size is important and keeping high vel. is important) but might be a non-factor for the next person. Again it’s all about balancing the trade-offs to get best solution for a specific individual’s needs and intended use.
 
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