Gas block closed vs open velocity loss or gain

[BobbyJ]

I got to thinking about this and have found only one article on it but it was 6.5 not 223/5.56 .

For people that have an adjustable gas block have you ever fully closed it and took the velocity then fully open it and retest. Wondering how much it actually drops the velocity or the range.

The the test I saw it was 28 fps loss vs closed.
It also might depend on the gas system and barrel length.
ie. rifle system 12" to the port and then 8 to 12" to the end of the barrel (depending on barrel length)

Just curious as to the effect on velocity in various combinations. I might get an adjustable block, curious to the results on velocity and on felt recoil. It seems most people should get a little velocity boost by turning down a over gassed AR.

Edit: Just saw this video

While he never tested fully closed (Simulate a bolt gun) he did find around 70 fps using a restricted adjustable gas block vs fully open.

 

[The boogalorian]

I built an ar chambered in 6arc a year ago. I had never put an ar together from parts and you could say it was a rough learning experience. I was shooting factory ammo for the brass to reload. It was a couple of range visits before I had the gun cycling reliably. Since then I have been experimenting with different loads. Last weekend I revisited my bolt, buffer and gas block settings. I use a light weight bolt with a JP captured spring. I removed the tungsten buffer weights and replaced them with the steel weights then tried different spring and gas block settings to reduce the recoil while maintaining reliable cycling. I was previously getting fired brass with lots of soot on the outside of the case but since the additional tuning sessions the soot on the outside of the case has been noticeably reduced.
Since the tuning I have not measured the velocity. Next time I bring that gun to the range I will chrono some loads and see if I can see a difference. It seems reasonable that lock time of the bolt and the dwell time would effect the velocity, just not sure how much of an effect it will have. I am interested in what others have to say about it.

 

[jelenko]

My understanding is the bullet has left the barrel before the bolt has unlocked.

 

[243winxb]

Vent barrel= less velocity. The closer to the chamber, more velocity drop.
Larger hole more velocity drop.

The sooner gas is vented, less recoil.

Port pressure must remain high enough to work the action.

 

[David Milisock]

I got to thinking about this and have found only one article on it but it was 6.5 not 223/5.56 .

For people that have an adjustable gas block have you ever fully closed it and took the velocity then fully open it and retest. Wondering how much it actually drops the velocity or the range.

The the test I saw it was 28 fps loss vs closed.
It also might depend on the gas system and barrel length.
ie. rifle system 12" to the port and then 8 to 12" to the end of the barrel (depending on barrel length)

Just curious as to the effect on velocity in various combinations. I might get an adjustable block, curious to the results on velocity and on felt recoil. It seems most people should get a little velocity boost by turning down a over gassed AR.

Edit: Just saw this video

While he never tested fully closed (Simulate a bolt gun) he did find around 70 fps using a restricted adjustable gas block vs fully open.

There's no free lunch, velocity loss is just one of the prices paid for auto loading.

 

[BobbyJ]

There's no free lunch, velocity loss is just one of the prices paid for auto loading.

I agree, I was just wondering how much of a loss open vs shut. It looks like it could be from 25-60 fps range. The best firing semi I had was probably my Ruger 10/22 stainless. SKS, Garrannds, M4s etc were not close to a bolt in accuracy.

I will say this AR experiment with the Aero upper is more accurate than expected for a semi. Its over gassed and shoots any ammo under 1 moa. Well so far it has.

If I had a adjustable gas block I could shut the gas off 100 percent or close and use it like a bolt gun. Just wondering how much a fps bump I will get.

 

[BobbyJ]

There's no free lunch, velocity loss is just one of the prices paid for auto loading.

Whats this? No free lunch? Can I at least have my cake and eat it too?

 

[BobbyJ]

I built an ar chambered in 6arc a year ago. I had never put an ar together from parts and you could say it was a rough learning experience. I was shooting factory ammo for the brass to reload. It was a couple of range visits before I had the gun cycling reliably. Since then I have been experimenting with different loads. Last weekend I revisited my bolt, buffer and gas block settings. I use a light weight bolt with a JP captured spring. I removed the tungsten buffer weights and replaced them with the steel weights then tried different spring and gas block settings to reduce the recoil while maintaining reliable cycling. I was previously getting fired brass with lots of soot on the outside of the case but since the additional tuning sessions the soot on the outside of the case has been noticeably reduced.
Since the tuning I have not measured the velocity. Next time I bring that gun to the range I will chrono some loads and see if I can see a difference. It seems reasonable that lock time of the bolt and the dwell time would effect the velocity, just not sure how much of an effect it will have. I am interested in what others have to say about it.

You sound like a natural, like to tinker and try stuff.

"I was previously getting fired brass with lots of soot on the outside of the case but since the additional tuning sessions the soot on the outside of the case has been noticeably reduced."

If you are getting dirty cases it can be from a few sources. I'm guessing the fix was you cleaned a dirty chamber which reduced soot.

- Dirty chamber that does not seal well
- Weak loads that do not fully expand the brass.
- old brass not sealing well.
- Buffer or spring (I don't think this is a cause)

When I upped loads for my Bolt 450 BM the slight factory sooting around the cases went away as I upped the charges. (they are under loaded by 20,000 psi)

Sounds like you are thinking that too light a buffer/spring = soot on the cases. Not so sure that is the actual cause.

Were you downloading your ammo too? I would look at the chamber and loads first. If you are using factory ammo and it goes from clean cases to dirty cases by changing you buffer spring that is another thing. The bolt will not move until enough gas passes into the BCG to unlock it.

 

[David Milisock]

I agree, I was just wondering how much of a loss open vs shut. It looks like it could be from 25-60 fps range. The best firing semi I had was probably my Ruger 10/22 stainless. SKS, Garrannds, M4s etc were not close to a bolt in accuracy.

I will say this AR experiment with the Aero upper is more accurate than expected for a semi. Its over gassed and shoots any ammo under 1 moa. Well so far it has.

If I had a adjustable gas block I could shut the gas off 100 percent or close and use it like a bolt gun. Just wondering how much a fps bump I will get.

Auto loaders today especially the AR are in my opinion are suffering from the modular assembly by less than knowledgeable assemblers. There's a tremendous amount of thought that goes into a functional self loading firearm. Robust designs fair better then marginal designs but it's to the point where I would never buy a used AR today.

As an example the 6MM ARC is a very functional design but marginalin terms of bolt thrust, I never read of bolt issues in rifles and loads that strictly adhered to proper designs. As far as I've seen there has been no unusual bolt failures in a 6 ARC, no more than 63,000 PSI 5.56 loads.

However they regular occur and 100% of the time that I've seen it was something that altered extraction timing, gas port placement or improper powder charges, sometimes marginal load over charges.

So, mess with an auto loader at your own risk.

 

[BobbyJ]

Auto loaders today especially the AR are in my opinion are suffering from the modular assembly by less than knowledgeable assemblers. There's a tremendous amount of thought that goes into a functional self loading firearm. Robust designs fair better then marginal designs but it's to the point where I would never buy a used AR today.

As an example the 6MM ARC is a very functional design but marginalin terms of bolt thrust, I never read of bolt issues in rifles and loads that strictly adhered to proper designs. As far as I've seen there has been no unusual bolt failures in a 6 ARC, no more than 63,000 PSI 5.56 loads.

However they regular occur and 100% of the time that I've seen it was something that altered extraction timing, gas port placement or improper powder charges, sometimes marginal load over charges.

So, mess with an auto loader at your own risk.

Not sure what you were trying to convey but sounds like you are saying some ARs use sub standard parts.
Of all the uppers I looked at the Aero Enhanced seems to be the strongest for AR and bull barrel. Very good engineering and the barrel fit perfectly with ZERO slop. And you can tighten the barrel nut without worry of lining up the gas key. I would buy the same one again (actually I am) just with a different color and gas block.

 

[David Milisock]

Not sure what you were trying to convey but sounds like you are saying some ARs use sub standard parts.
Of all the uppers I looked at the Aero Enhanced seems to be the strongest for AR and bull barrel. Very good engineering and the barrel fit perfectly with ZERO slop. And you can tighten the barrel nut without worry of lining up the gas key. I would buy the same one again (actually I am) just with a different color and gas block.

The AR is simply a name, it's the assemblers that make the choices, sometimes good choices sometimes bad choices. Below is an example, it's in 6ARC but similar things are happening in mass with all cartridges in the AR platform.

I had an Aero Precision built. I called Brownells and talked to their tech, he builds their assembled rifles and was an armorer. I bought a complete heavy barreld upper, they come to Brownells assembled with matched parts and the proper position for the gas port. I got an Aero lower and followed his suggestions for a 2 stage match trigger, iron sights, buffer and stock. I had my gunsmith of 40 years assemble and check the headspace. It shoots 1/2" groups at 100 yards with Hornady 105 grain match. I have faith that with factory ammo or my carefully created reloads I'll get the same performance innterms of reliability from it as my Colt AR15 A2.

There's no getting around the fact that the 6MM ARC has higher bolt thrust than the 5.56. If you follow this forum you see AR's of the same cartridge BUT, a lightweight bolt carrier was used, this changes the extraction timing, the pressure at extraction is higher then designed. Buffers are not standard, this also changes timing. Gas ports are not placed as designed or adjustable ports are used. All parts are chosen and may come from many manufacturers with unknown practical tolerances. Handloads are used and velocities are peak and powders are not what the cartridge was designed for.

The results are varied those who make good choices or those who are lucky get good rifles. Thos who make bad choices or who are unlucky get bad rifles. In any case those who reload carefully and have a good rifle do well, those who are not careful reloaders break rifles good or bad rifled and may hurt people.

The AR market unfortunately is a serious buyer beware market regardless of cartridge but especially for the cartridges with high bolt thrust.

 

[HSMITHTX]

I have seen AR 'armorers' at an AR platform rifle company assemble a rifle from parts and fire it in under 7 minutes, stripped upper and lower, floating handguard etc. 7 minutes was the expected time, one of the guys could get under 4 min occasionally. Commodity rifles and low end at that, but how much of anything was checked? These rifles I saw being 'built' went from the bench to a test fire of 3 rounds, if it went bang 3 times in the box it went. That was the sum total of QC, bang bang bang means good to go. Commodity AR platform rifles are categorically garbage, in the parts used and the results obtained. Thankfully, or not, it is one of the most forgiving semi-auto platforms out there for function, it will likely fire if close to semi-ok.

I have done a lot of testing on AR platform rifles in 223/5.56 for gas port location, size, bleed vs restriction gas blocks, etc. I was of the opinion that a lot of what was being done was because that was the way it has always been done or by God Colt did it that way. Some of that is true and some isn't I think. And the 'facts' according to me are that it all depends. It depends on load, bullet weight, powder used, pressure, pressure curve, barrel length, etc.

If you think about it the location of the gas port determines how much pressure the port will tap from the barrel. Pressure generally diminishes as the bullet travels down the bore, move the port toward or away from the muzzle and for any given load you can port more or less pressure. The size of the port will determine the volume. How much barrel is beyond the port will determine the gas pulse width or how long the gas pressure is applied to the tube and BCG at the pressure ported and the volume allowed by the port size. Let's say you dial all of this in perfectly for 223 with 69MK and Varget. Excellent shooting rifle and you love it. Now you change to a 55 grain bullet and BLC2, or a 77 grain bullet and stay with Varget and everything changes. Some to a small degree and the effects are negligible. Some might change more dramatically because you have changed the pressure at the port, faster powders will generally have less pressure at the port than slower powders. A heavier bullet is generally a slower velocity bullet at higher port pressure (because you are using slow powder) and results in a higher pressure at the BCG and a longer duration gas pulse. How much effect this has is determined by how much things change from the design parameters, in general the rifle will still run if it was a quality rifle to begin with but you can see pretty dramatic differences in accuracy and recoil impulses.

Cheap barrels make for interesting experiments and 12-15 years ago I played with a bunch of different configurations. Get a 20" straight contour barrel and turn to diameter for your gas block of choice, drill a gas port at the location you want to test and shoot it. Tap the gas port hole and plug it WITHOUT restricting the bore and do it again at a different location, as many times as you want. Then cut the barrel down and do it all again for as many lengths as you want to test. I was shooting action games at the time and for me recoil impulse and sight displacement during rapid fire was my main concern after reliability. For me I am in the camp of mid-gas for 14.5-16 inch barrels. For an 11" I strongly suspect carbine gas is required but I don't have any experience with them other than shooting a few. I like rifle gas for 18's and rifle plus one or two for 20" using 69+ grain handloads. 72-77 are where the plus two really shines on a 20" IMO but it needs powders on the slow side and the loads pushed near max. I never tested longer than 20" in 223/5.56 but I suspect a +2 is plenty to 24" maybe longer. ALL of my AR rifles except two have adjustable gas blocks, you cannot move the port once it's drilled (actually you can but for the sake of discussion let's take it off the table) but I can easily manipulate the size of the port so why wouldn't you have that option given that good quality steel adjustable blocks are available and aren't super expensive? I've come to appreciate the bleed style blocks like Superlative vs the restriction type blocks, I haven't seen a bleed block with a stuck adjustment yet but I have seen quite a few restriction type blocks get tossed. I never measured velocity as a criteria when I was testing, my opinion is that it will be what it will be and if the load is reasonable it will be sufficient so long as it is predictable and consistent. I am way more worried about getting the ES down under 30 and SD low than I am about whether or not I can squeeze 20 FPS out of the load or gas block setting. MOA was plenty accurate for anything I have ever done with a semi-auto rifle. I've never had an interest in subsonic application of the AR type rifle either, so a closed gas block or grenade setting has never interested me either as mine are all semi-auto use so I require them to cycle fully on every shot.

There are lots of ways to make your AR platform rifle your own, and to make it shoot your loads the way you want it to, and it's fun so by all means test what you want to test. So long as the load is reasonable and the bore isn't restricted go for it I say.

 

[jelenko]

I have seen AR 'armorers' at an AR platform rifle company assemble a rifle from parts and fire it in under 7 minutes, stripped upper and lower, floating handguard etc. 7 minutes was the expected time, one of the guys could get under 4 min occasionally. Commodity rifles and low end at that, but how much of anything was checked? These rifles I saw being 'built' went from the bench to a test fire of 3 rounds, if it went bang 3 times in the box it went

I get that on the production line the assembly goes quickly. But that doesn't say anything about the engineering/design that went into the rifle first.

PS. 7 minutes is fast - but, I think it also demonstrates that the AR is similar to a Leggo set - just assemble.
If you start with quality parts and have taken into consideration the loads you'll be using, there really isn't much else to it. One could square the face of the upper and dab some sleeve loctite around the barrel extension that might improve accuracy a bit.

 

[BobbyJ]

The AR is simply a name, it's the assemblers that make the choices, sometimes good choices sometimes bad choices. Below is an example, it's in 6ARC but similar things are happening in mass with all cartridges in the AR platform.

I had an Aero Precision built. I called Brownells and talked to their tech, he builds their assembled rifles and was an armorer. I bought a complete heavy barreld upper, they come to Brownells assembled with matched parts and the proper position for the gas port. I got an Aero lower and followed his suggestions for a 2 stage match trigger, iron sights, buffer and stock. I had my gunsmith of 40 years assemble and check the headspace. It shoots 1/2" groups at 100 yards with Hornady 105 grain match. I have faith that with factory ammo or my carefully created reloads I'll get the same performance innterms of reliability from it as my Colt AR15 A2.

There's no getting around the fact that the 6MM ARC has higher bolt thrust than the 5.56. If you follow this forum you see AR's of the same cartridge BUT, a lightweight bolt carrier was used, this changes the extraction timing, the pressure at extraction is higher then designed. Buffers are not standard, this also changes timing. Gas ports are not placed as designed or adjustable ports are used. All parts are chosen and may come from many manufacturers with unknown practical tolerances. Handloads are used and velocities are peak and powders are not what the cartridge was designed for.

The results are varied those who make good choices or those who are lucky get good rifles. Thos who make bad choices or who are unlucky get bad rifles. In any case those who reload carefully and have a good rifle do well, those who are not careful reloaders break rifles good or bad rifled and may hurt people.

The AR market unfortunately is a serious buyer beware market regardless of cartridge but especially for the cartridges with high bolt thrust.

Yea I can imagine there are issues with so many parts suppliers and people of various levels of skill/knowledge building 6mm ARC. I have a couple of Colts and while they are built well and just run. The AR with 24" bull barrel built is more accurate. The Colts were never meant to be super accurate they were built to run reliably. Adding a adjustable gas block only increases complexity and decreases reliability but easier to tune for various loads.

 

[BobbyJ]

I've come to appreciate the bleed style blocks like Superlative vs the restriction type blocks, I haven't seen a bleed block with a stuck adjustment yet but I have seen quite a few restriction type blocks get tossed. I never measured velocity as a criteria when I was testing, my opinion is that it will be what it will be and if the load is reasonable it will be sufficient so long as it is predictable and consistent. I am way more worried about getting the ES down under 30 and SD low than I am about whether or not I can squeeze 20 FPS out of the load or gas block setting.

On the gas blocks I choose the odin, its design is one that does not look like it will get frozen and I do like that you get a boost in fps for free.
On my 24" with the standard rifle gas system it makes the 24" act more like a 22" due to excess gas. With the bleed off system you don't get any boost in fps with restriction type your gun acts more like a true Bolt in terms of expected velocities.
So I ordered the odin to see how the free boost of 60-80 fps works. Once I set it I probably will not mess with it so if it were to carbon freeze up I won't notice unless I start to tinker with it.

 

[David Milisock]

Yea I can imagine there are issues with so many parts suppliers and people of various levels of skill/knowledge building 6mm ARC. I have a couple of Colts and while they are built well and just run. The AR with 24" bull barrel built is more accurate. The Colts were never meant to be super accurate they were built to run reliably. Adding a adjustable gas block only increases complexity and decreases reliability but easier to tune for various loads.

Mil Spec rifles are over gassed to function in bad environments. Gas auto loader systems were made for a specific barrel length, port size/position, bullet weight, powder burn rate, pressure curve and mechanical cycling system. Mechanical autoloader systems too but they tend to be less sensitive.

Screw with any of it and pressure on the bolt lugs during unlocking is in general the first this affected but it's the hardest thing to see. Bad JuJu for the 6ARC!

I wouldn't call auto loaders 1 trick ponies but they are limited trick ponies. However the more you treat them like 1 trick ponies the better they function and when they function they're the finest general purpose rifle a family can have.

 

[BobbyJ]

Mil Spec rifles are over gassed to function in bad environments. Gas auto loader systems were made for a specific barrel length, port size/position, bullet weight, powder burn rate, pressure curve and mechanical cycling system. Mechanical autoloader systems too but they tend to be less sensitive.

Screw with any of it and pressure on the bolt lugs during unlocking is in general the first this affected but it's the hardest thing to see. Bad JuJu for the 6ARC!

I wouldn't call auto loaders 1 trick ponies but they are limited trick ponies. However the more you treat them like 1 trick ponies the better they function and when they function they're the finest general purpose rifle a family can have.

Agree if you stick with what they are made to do 55 and 62 grain fixed gas.

6ARC I have never shot, are they running 6ARC with 223 type parts? Meaning its not 308 strength bolts, lugs, chambers? I could see that to be a accident waiting to happen if true.

 

[243winxb]

The M16A1 rifle & carbine had different size gas ports, at different locations.

When buying parts, barrel & bolt should be of same brand/manufacture. Better chance of getting the correct headspacing.

At the range, i have seen rifles assembled on the firing line & shot without checking anything. Parts were still in plastic bags, opened & assembled.

 

[David Milisock]

Agree if you stick with what they are made to do 55 and 62 grain fixed gas.

6ARC I have never shot, are they running 6ARC with 223 type parts? Meaning its not 308 strength bolts, lugs, chambers? I could see that to be a accident waiting to happen if true.

The 6MM ARC uses the AR15 with an opened up bolt face, a type 2, 6.5 Grendel bolt I believe and of course 1 6MM ARC barrel.

The bolt lugs are smaller, the face bigger to accommodate the larger diameter case head. Pressure is limited to 52,000 PSI, but Hornady did this with a very specific Pressure curve generated by LeverEvolution powder.

So all the mechanical aspects are critical because the stress levels are toward the upward maximum, so precision in loads and duplicating designed mechanics are a must.

 

[BobbyJ]

The 6MM ARC uses the AR15 with an opened up bolt face, a type 2, 6.5 Grendel bolt I believe and of course 1 6MM ARC barrel.

The bolt lugs are smaller, the face bigger to accommodate the larger diameter case head. Pressure is limited to 52,000 PSI, but Hornady did this with a very specific Pressure curve generated by LeverEvolution powder.

So all the mechanical aspects are critical because the stress levels are toward the upward maximum, so precision in loads and duplicating designed mechanics are a must.

Wonder if 6arc has more bolt thrust than people that shoot 450 BM from an AR. I think those are like 38,000 PSI though but a 308 sized case head and heavier 250-275gr bullet. I shoot those from a bolt made for it at 52,000 psi.