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Sub Sonics

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Sub Sonics was created by Flowhanan

I have a DPMS ap4 LR308, with a YHM can. I use the can mostly on a bolt gun but i was curious how to get the auto gun to cycle with sub sonic loads. Having trouble finding much info.
11 years 1 month ago #4941

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Replied by Flowhanan on topic Re:Sub Sonics

The search continues for information
11 years 1 month ago #5001

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Replied by Hot Lead Zapper on topic Re:Sub Sonics

Flowhanan wrote:


Adjustable gas blocks is the first place to start. AR10s are known for there over gas pressure issues in general and as you know a suppressor will increase the backpressure causing reliable function failure. JP Rifle Company has most experience and line of products to address this issue. I
10 years 10 months ago #5732

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Replied by Hot Lead Zapper on topic Re:Sub Sonics

Flowhanan wrote:


Sorry, I
10 years 10 months ago #5735

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Replied by 6-Sigma on topic Re: Sub Sonics

Ever find anything to work for you?
9 years 7 months ago #10306

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Replied by Hot Lead Zapper on topic Re: Sub Sonics

I’ll post some pictures of items that has helped me to address over and under pressures for the ID system of AR platforms, especially for suppressor and SBR issues.

Most of my experiences started with AR15/M16 Shorty/M4 - 223 based caliber and continues towards the 308 AR10.

Tim LaFrance twin tube expansion chamber solves over pressure for suppressed AR15 16” barrels as well as short pulse pressure for full-auto and semi-auto SBRs. SBR length barrel require larger barrel gas ports and 16” barrels don’t. The LaFrance system is difficult to find because Tim retired from SureFire last year after designing their suppressors. Chris at KNS buys several NIB LaFrance twin tubes every year and offers them once a year. www.knsprecisioninc.com/contacts

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JP Ind adjustable gas blocks works to limit or increase gas pressures as needed. They also offer adjustable gas blocks for 308 mid diameter barrels. www.jprifles.com/1.4.6_gs.php

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Armalite’s Super SASS adjustable gas block is absolutely incredible gas system. It’s simple to use and researched for the best results. Personally I never mounted one onto a different manufacture’s AR 308 rifle or barrel, but I would guess in could be done by a competent gunsmith. I believe the only issue of concern would be the gas key link up with the Armalite’s adjustable gas tube…? www.armalite.com/ItemForm.aspx?item=1010...73-9e49-8b550aadc53b

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Hydraulic buffers alone will not solve pressure issues, but as an added feature accommodates adjustable gas block affects for lower muzzle rise and felt recoil. Hydraulic buffers will increase the range of adjustment for gas pressures, especially the ITT Enidine Buffer. www.midwayusa.com/find?userSearchQuery=Enidine+buffers

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One of the most important issues of concern for subsonic center fire rifle ammo is barrel twist rate...! 1 - 10 rate is minimum for 180gr to 220gr projectiles and some of my shooting buddies have gone to 1 - 8 twist rate to stabilize above 220gr projectiles.
9 years 7 months ago #10316

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Replied by Flowhanan on topic Re: Sub Sonics

Nearly a year after I posted a comment return reply's finally arrive. I still haven't resolved my old question. What has to be done to an LR-308 to make the rifle cycle shooting sub-sonic ammo with a suppressor?? Question no. 2 should be an easy on. I installed a MAG-PUL fixed adjustable stock. My bolt now has problems closing all the way. forward assist has to be used. Do I need a different buffer or another type of spring???
9 years 7 months ago #10334

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Replied by 6-Sigma on topic Re: Sub Sonics

Are you using a mil spec spring and buffer tube? I switched to the mil spec stuff after several issues using a commercial set, on my M4 when using the mil spec stuff i can feel a faster return to battery position. Might be a good conversation Pm me your number I'll give you a call
9 years 7 months ago #10337

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Replied by Flowhanan on topic Re: Sub Sonics

6-sigma. just sent off a PM.
9 years 7 months ago #10338

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Replied by JeffSavannah on topic Re: Sub Sonics

Not to talk over the advice you are getting otherwise, most of this is over my head, but I know that if you are using sub sonic ammo with a can on an LR308, assuming you have no buffer/spring/bolt/return to battery problems... you can simply tune your powder charge so that you get adequate pressure to reliably cycle the bolt. You will get overpressure from the can, but underpressure from the light charged sub sonic ammo. Bullet weight will of course play an important part and tuning that can also contribute to finding the perfect pressure situation. If you are not handloading, then getting specific data from the manufacturers about the charge they are using for a specific sub sonic offering will be a good place to start... but if I were you I would get familiar with what general area of powder charge you need, and have some custom ammo ordered from one of the companies that make ammo to customer spec. I am NOT an expert on any of this, though I wanted to own an adjustable gas block on my Noveske with plans to use a can, but decided with the rifle that became available to me to put that off for another project for another day. I ask anyone who knows more than me to please correct me if this advice is not viable, but from my basic knowledge of subsonic-can use, I believe what I'm saying is generally correct. Please share with us your solution, if found. Good luck. *edit: meant to include that the adjustable gas block is of course the first and more important part to play in this. You will need, as advised by someone else, a five position rather than a three position, adj. gas block. This will give you an off, suppressed, and standard gas port size, plus two additional positions, one between off and suppressed, and one between suppressed and standard. For sub sonic purposes, the position between suppressed and standard would be the closest to your needs, it will allow more pressure to make up for the light charge, but not so much as to cause overpressure from the can. This will at least get you closer so that your cartridge tuning will be a tighter range and closer to standard. It might actually be all you need. These five position blocks may keep you from having to do anything else. The JP advice should be the first place you start.
Last edit: 9 years 7 months ago by JeffSavannah.
9 years 7 months ago #10345

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