Do I want a 7.5" or 10+" barrel SBR?

[CJ]

A 7.5" has a substantial loss in velocity on .223. You'll get around 2200fps for a 55gr, which is considerable. And like mentioned before almost no suppressor is warrantied for that short of a barrel. HOWEVER, this mostly applies to 5.56mm cans, because they are built for lower pressures. Most of your .30 cal cans are built for these pressures and given the larger bore diameter they are less affected by short barrels/blasts. I would refrain from putting a 5.56 can on a 7.5", but not a .30 cal can - but neither on full auto. I have a 7.5" .223 from redxarms in the mail now, I want it because it's nice and convenient for hunting. I'm going to run a piston on my 7.5" with a pistol gas location because you get vicious impingement at that short of a gas sample, but reliability should not be a problem at all. Like Keith mentioned some of the 11.5's and barrels in that range can have issues with gas blocks but that isn't a concern on a pistol length, as the gas block is 3.5" behind the muzzle. And they are loud as shit as mentioned before, most everyone goes with a linear compensator to prevent your ears from ringing inside your head phones.

edit: changed your thread title to a question.

[inline 6]

Thanks CJ, I am going to order the same one from REDX on Friday when I get paid. Worst case I hate it and order a longer barrel down the road.

[ThreeFingerPete]

I understand the real difference between the two I was more wondering why it is acceptable by most to have a 7.5" pistol but it is frowned upon for a rifle.

Seriously? I just pretty much summed it up. 7.5" guns are fine for a toy role, but they tend to be finicky, they lose a lot of velocity compared to a 11.5", the muzzle blast is rowdy as fuck, etc. Just like my Mini Draco. It's hilarious and fun to shoot, but it's very obviously a novelty.

Put a stock on a rifle, and now for some reason people want to be able to do rifle stuff with it. Hunt, home defense, use suppressors, etc. All of which are about 100x more easily done with a 11.5" gun.

If you want the 7.5" gun, get one!

[ThreeFingerPete]

I built a 11.5" pistol with a carbine gas system and found out that the 10.5" would not allow enough space between the flash hidder/suppressor mount and the gas block for my G5 to go on. So it depends on your suppressor mount, gas block location/type, etc on what you need to run.

an 11.5" also has something like 40% more dwell time so they run a lot better.

[David]

My 11.5" with a brake is retarded dumb loud. I couldn't imagine 7.5".

[CJ]

an 11.5" also has something like 40% more dwell time so they run a lot better.

Dwell time is distance from the gas port to the muzzle. It's all relative to the gun and where its gas port is located. Dwell time is a commonly referenced factor but it's very misunderstood. Dwell time is offset by pressure, the less pressure the more dwell time is necessary, and with more pressure less dwell time is needed. 7.5" have no issues with reliability.

[ThreeFingerPete]

Dwell time is distance from the gas port to the muzzle. It's all relative to the gun and where its gas port is located.. However dwell time is commonly referenced element but very misunderstood. Dwell time is offset by pressure, the less pressure the more dwell time is necessary, and with more pressure less dwell time is needed. 7.5" have no issues with reliability.

I know what Dwell time is, and the more dwell time the gun has, the smaller the gas port needs to be(all else being similar, not including changes in gas port location) , the less violent the recoil, by far cleaner. I would run the longest gas system that would function reliably on any AR I could. The benefits are a lot smoother shooting gun. 7.5"s can run just fine, so can 10.5"s, but it takes tuning and while YOU may be capable of doing it, it doesn't mean that everyone else can/will.

Additionally, that force is needed because the brass is still expanded in the chamber and will have to be forcibly removed by the extractor. There are issues. Just because they're issues that can be worked through doesn't mean that they don't exist. Most people want to buy a gun and shoot it.

[CJ]

I know what Dwell time is, and the more dwell time the gun has, the smaller the gas port needs to be(all else being similar, not including changes in gas port location) , the less violent the recoil, by far cleaner. I would run the longest gas system that would function reliably on any AR I could. The benefits are a lot smoother shooting gun.

My point was their dwell time has no bearing on their reliability or how they run - it's all if they are matched correctly to the buffer spring. When you don't have that matched correctly, you're going to have excessive recoil.

[ThreeFingerPete]

My point was their dwell time has no bearing on their reliability or how they run - it's all if they are matched correctly to the buffer spring. When you don't have that matched correctly, you're going to have excessive recoil.

Increased dwell time will allow a greater variety of ammunition to be fired reliably, which is a very real concern. It will also make a smoother recoil cycle, which tends to be less problematic. A longer moderate pulse is more consistent than a shorter harder pulse.

[CJ]

Increased dwell time will allow a greater variety of ammunition to be fired reliably, which is a very real concern. It will also make a smoother recoil cycle, which tends to be less problematic. A longer moderate pulse is more consistent than a shorter harder pulse.

My personal experience shows it's all in how you set it up. A lot of that gets perpetuated on the net without anyone actually having some real world experience with it. As a matter of fact, most of the people I hear that from are people who don't even know how to build an AR15. If you set your gun up correctly, you can minimize any recoil and still have a reliable gun. It's all conjecture and theory - I personally find harder cycling guns to be more reliable when they get dirty, and lower pressure high dwell guns to jam/fail to return to battery in the same situation. I will agree that a longer dwell time can to a very small degree lessen the type of recoil, but it isn't going to make it run better or be more reliable. I would say a harder cycle is more reliable.

[ThreeFingerPete]

My personal experience shows it's all in how you set it up.

Agreed 100%.

A lot of that gets perpetuated on the net without anyone actually having some real world experience with it.

As is the case with almost everything.

If you set your gun up correctly, you can minimize any recoil and still have a reliable gun.

No disagreement there!

I personally find harder cycling guns to be more reliable when they get dirty, and lower pressure high dwell guns to jam/fail to return to battery in the same situation.

Absolutely,because they are generally very over-gassed.

I will agree that a longer dwell time can to a very small degree lessen the type of recoil, but it isn't going to make it run better or be more reliable. I would say a harder cycle is more reliable.

You will have better extraction... Until small parts break.

[CJ]

You will have better extraction... Until small parts break.

I've never had a small part break on any AR15 other than my M16 which the forward assist arm broke. And that has to be tens of thousands of rounds on various guns. This is theory and speculation, but I don't buy it.

How much dwell time does your louderer rifle have?

[ThreeFingerPete]

I've never had a small part break on any AR15 other than my M16 which the forward assist arm broke. And that has to be tens of thousands of rounds on various guns. This is theory and speculation, but I don't buy it.

How much dwell time does your louderer rifle have?

Not much, and its tuned down ridiculously low with the adj gas block. It's an 18" rifle length gas.

[Chopped54]

Just got back from Ident and they said their 5.56 can is warranted at 7.5" full auto.

[svauto-erotic855]

I would not do a SBR with a 5.56x45. That cartridge really needs a 24" barrel and anything shorter really puts a dampener on the performance. A 6.5mm Grendel, 7.62X39 or 6.8mm SPC would be a much better choice for a SBR. I'd like one with a 12" barrel as that length doesn't totally kill performance.