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#6169291 - 02/13/12 08:30 AM Re: Carbine vs mid length gas block [Re: Take_a_knee]
TWR Online   content
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Registered: 11/21/06
Posts: 5252
Originally Posted By: Take_a_knee
TWR, a 16in and a 14.5 M4 have the same gas port pressure, if the guns are other wise configured the same. The longer barrel has a slightly lower pressure at the crown because it has a longer dwell time.

A 16in barrel also pushes M855's effectiveness out about another 50-75 yd. By that I mean it'll break at the cannelure like M193. Mid-length 16in is the way to go.


Yes, if you've ever seen a bayonet on a 16" carbine gas gun, you'd understand stubby. I'm pretty familiar with the 14.5 and 16" carbines gas systems but the longer barrel past the gas port actually increases pressure. That's why so many say the 14.5" with a mid length gas system is so smooth to shoot, less dwell time.

I've chronographed Colt 14.5", 16" and 20" barrels with m193 and found there's 106fps difference between the 16 and 20 and 101fps difference between the 16 and 14.5" (averaged of the 3 barrels I was testing). The 16" with a midlength system would be a better gun but it took Colt 15 years to convince the DOD to add a stiffer insert and extractor spring to the TDP, I'm afraid we'll never see a 16" M4 issued but as you pointed out, it would extend the range.

And thanks for explaining the bolt over base.

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#6173120 - 02/14/12 03:49 AM Re: Carbine vs mid length gas block [Re: Take_a_knee]
bbassi Offline
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Registered: 02/20/09
Posts: 5819
Thanks for the info. It was very helpful.
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#6195064 - 02/19/12 02:34 PM Re: Carbine vs mid length gas block [Re: bbassi]
bbassi Offline
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I'm going to bring this back up because I have another question -

I'm looking at stocks/buffer combos for this set up and I'm wondering what I need if I go with the mid-length gas system. I prefer the solid A2 style stocks, but I have no idea what I need for a buffer/spring if I go with the A2. Will the standard A2 like RRA sells work well with the mid-length?

RRA
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#6195080 - 02/19/12 02:40 PM Re: Carbine vs mid length gas block [Re: bbassi]
TysonT Offline
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Registered: 05/23/10
Posts: 658
Loc: Iowa
Gas system has no effect on Buffer Spring, Buffer weight/Style, and stock selection. If you are going with a standard A2 butt stock, you will need a Rifle length (also called standard) Buffer tube, buffer spring, and a Rifle buffer.

So basically if you do the A2 stock, avoid anything butt stock related that says Carbine or Collapsible and you'll be okay.
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#6195850 - 02/19/12 05:48 PM Re: Carbine vs mid length gas block [Re: TysonT]
Take_a_knee Offline
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Registered: 09/10/09
Posts: 12291
Originally Posted By: TysonT
Gas system has no effect on Buffer Spring, Buffer weight/Style, and stock selection.


If that is the case (it isn't) then those guys at JP rifles are idiots and/or shysters, and I really don't think so.

EVERYTHING about an AR's gas/operating system affects everything else.

I just put an upper together, 18in mid-length. Runs great on green tip. Bolt won't lock to the rear on ANY milder load, with a rifle or carbine lower. Change to a lighter buffer in a rifle lower and it'll run anything, even some really mild reloads.

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#6195928 - 02/19/12 06:06 PM Re: Carbine vs mid length gas block [Re: Take_a_knee]
bbassi Offline
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Registered: 02/20/09
Posts: 5819
So Take a knee, are you saying you are running a carbine buffer and spring in a rifle length buffer tube, or just the the carbine buffer with a rifle spring?
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#6195978 - 02/19/12 06:16 PM Re: Carbine vs mid length gas block [Re: Take_a_knee]
TysonT Offline
Campfire Regular

Registered: 05/23/10
Posts: 658
Loc: Iowa
Originally Posted By: Take_a_knee
Originally Posted By: TysonT
Gas system has no effect on Buffer Spring, Buffer weight/Style, and stock selection.


If that is the case (it isn't) then those guys at JP rifles are idiots and/or shysters, and I really don't think so.

EVERYTHING about an AR's gas/operating system affects everything else.

I just put an upper together, 18in mid-length. Runs great on green tip. Bolt won't lock to the rear on ANY milder load, with a rifle or carbine lower. Change to a lighter buffer in a rifle lower and it'll run anything, even some really mild reloads.


No, I'm saying that "in theory" if you have a complete upper, you can successfully mate it to a complete lower despite the length of barrel, gas system, or butt stock. And should be okay as long as you are running the correct spring, buffer, and buffer tube configuration for that particular butt stock (carbine or standard). Except in the case of carbines where a different weighted buffers may be necessary to get proper cycling in usually "non-standard" calibers or bullet weights that are either extremely heavy or light. That being said, there are always the odd ball case where something else may be effecting the complete operation of the entire weapon (your case for example... maybe) and the components won't work, but you might not know until you try it out either.

But in the OP's question he was asking specifically about the A2 butt stock configuration.

If I understand what you wrote, you have to run a lighter carbine buffer in a rifle lower (with rifle style spring, not carbine spring) to get it to fire all loads? Are you sure you don't have another issue with your upper, like the gas port in the barrel is too small or something in the gas system is not lined up correctly?


Edited by TysonT (02/19/12 06:57 PM)
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#6196269 - 02/19/12 07:22 PM Re: Carbine vs mid length gas block [Re: TysonT]
TWR Online   content
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Registered: 11/21/06
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"Change to a lighter buffer in a rifle lower and it'll run anything, even some really mild reloads."

I'm lost on this one as well, there is only one A2 stock buffer and spring (specialty buffers excluded) and it's for a rifle gas system. The middy has more pressure, if you have to go to a lighter buffer, something is terrribly wrong.

If you're running a carbine stock on a rifle gas system then, yes it can be tough to find the right combo.

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#6196286 - 02/19/12 07:25 PM Re: Carbine vs mid length gas block [Re: TysonT]
Take_a_knee Offline
Campfire 'Bwana

Registered: 09/10/09
Posts: 12291
Originally Posted By: TysonT
Originally Posted By: Take_a_knee
[quote=TysonT]Gas system has no effect on Buffer Spring, Buffer weight/Style, and stock selection.




If I understand what you wrote, you have to run a lighter carbine buffer in a rifle lower (with rifle style spring, not carbine spring) to get it to fire all loads? Are you sure you don't have another issue with your upper, like the gas port in the barrel is too small or something in the gas system is not lined up correctly?


It's a barrel from Montana Barrels so I doubt they screwed the gas port up but that's possible. I double checked the gas block, it the same as a friend has used on several guns so I don't think that is the issue.

Yes, rifle lower, non-full auto carrier with a rifle buffer with the weights removed, runs great.

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#6196320 - 02/19/12 07:32 PM Re: Carbine vs mid length gas block [Re: Take_a_knee]
TWR Online   content
Campfire Tracker

Registered: 11/21/06
Posts: 5252
They screwed the gas port size.

Remember the A2 stock runs a buffer spring 11.750"-13.5" with a buffer that weighs 5.2 oz. The gas port pressure is 13.5K psi in a rifle length system and runs fine as designed with a full auto carrier, millions of M-16A1's and A2's have proven this all over the world.

The middy system taps it's gas at 17K psi.

Think about this for a minute...

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