bullet speed difference gas operated semi auto vs bolt action?

[NomadS]

How much difference is there in bullet speed between a gas operated semi auto vs a bolt action? I would think the bolt action would shoot faster (if equal barrel length) since none of the gas is be diverted to work the bolt and therefore pushes the bullet faster. Is this the case?

Reason for asking: I am thinking about cutting down the barrel on a bolt action I have and want to use the same same subsonic rounds I use in my semi auto while continuing to keep the bullet subsonic. Want to know what length to cut the barrel.

Barrel twist is 1 in 10 in the bolt. 1 in 11.25 for the semi auto.

 

[traderdan]

The difference would be to small to notice in my opinion.

 

[j706]

I don't know the exact answer but I would think there would be very little difference. I assume that a bullet is still gaining speed as long as it is in the barrel under pressure (within reason) There has to be a point where it starts to go the other way ( slowing) depending on powder burn rate ect. I happen to really like short barrel rifles of any type. My most accurate gun is a 18" .308 bolt gun. My elevation dope from when it was 26" down to 18", at the ranges I shoot (400 yards max), was hardly enough to warrant noting in my log book.

I have never messed with sub sonic loads yet. But my simple mind tells me that the powder burn has peaked it's pressure in a very short distance. What barrel length is your semi auto? I am thinking you are at a minimum GTG at cutting to your semi auto's length. The barrel twist rate shouldn't make much difference IMO.

 

[NomadS]

Semi is 18 inches .. Bolt is currently 22

 

[j706]

Semi is 18 inches .. Bolt is currently 22

I wouldn't hesitate for a second to cut it down to 18". I don't know but I would find it highly unlikely that 4" less would take you over subsonic.

 

[Double T]

Less barrel=less velocity, not more so I'm kinda confused as to why it would be an issue?

Differences in velocity will vary from load to load and gun to gun regardless of barrel length.

 

[NomadS]

Less barrel=less velocity, not more so I'm kinda confused as to why it would be an issue?

Differences in velocity will vary from load to load and gun to gun regardless of barrel length.

The semi auto diverts some of the gas to cycle while the bolt action doesn't. I don't know if that increased amount of gas pushing the bullet will increase velocity or not. And if it does by how much? That's my question.

 

[kludge]

Before cutting your barrel, chonograph your load.

As a rule of thumb, you will gain/lose ~25fps/inch of barrel.

 

[HoughMade]

The semi auto diverts some of the gas to cycle while the bolt action doesn't. I don't know if that increased amount of gas pushing the bullet will increase velocity or not. And if it does by how much? That's my question.

And it's and interesting question which seems to have been lost in all the barrel length talk- back on point:

I heard somewhere that the velocity loss for the semi-auto is around 23 fps....but darned if I can remember where I heard that. I'll keep looking.

[edit]Some sources say 3%, one guy on a forum says he tested the same loads in barrels before and after he made the gass port and found no meaningful difference. Whatever difference there is, it does not seem to be a big one.

 

[NomadS]

Before cutting your barrel, chonograph your load.

As a rule of thumb, you will gain/lose ~25fps/inch of barrel.

So based on that, I should shoot the identical load through both guns and record my average chrono for both.

Then, assuming I do find the bolt action is indeed shooting faster than the semi auto decide how much to cut it down based on how much faster. If it is 100 fps then I should cut it down 4 inches, 75 fps faster = 3 inches, 125 fps = 5 inches.

Sound right?

 

[ROLEXrifleman]

I have and want to use the same same subsonic rounds I use in my semi auto while continuing to keep the bullet subsonic. Want to know what length to cut the barrel.

Don't over think it. Wack that barrel to the same lenth as your semi.

You already stated you know it's subsonic out of that length barrel. How you know I have no clue, but you say it is so....

Anyway, chop it to the same lenth and the bolt action will be less than 25fps faster due to no gas purge. This us in addition to any variance in the rounds themselves.

No One ever loads subsonics right at 1126 anyway.
So any variance in load and action type should still keep you under the magic number as long as the barrel lengths are kept the same as your verified subsonic gasser length.

 

[ROLEXrifleman]

Btw, what load are you shooting out of the gas gun? With that twist rate Im curious how well your bullets are stabilizing at subsonic speeds and what grain bullet your using.
Please let mr know, im very curious what the load is, what can your using and what kind if groups your printing.

 

[U.S. Patriot]

The gas does not actually fire the bullet though, it cycles the bolt. I think you are overcomplicating things.

 

[NomadS]

Btw, what load are you shooting out of the gas gun? With that twist rate Im curious how well your bullets are stabilizing at subsonic speeds and what grain bullet your using.
Please let mr know, im very curious what the load is, what can your using and what kind if groups your printing.

haven't yet. Waiting on the atf to approve the can. YHM titanium.

Planning on starting with 11.o grains of Trail boss and a hornady 180 gr round nose bullet. Will work down from there until subsonic. (have to buy/borrow a chronograph)

Won't put the can on until I get consistent, nice round holes on the target.

Any recommendations?

 

[NomadS]

The gas does not actually fire the bullet though, it cycles the bolt. I think you are overcomplicating things.

agree I might be over analyzing. But would rather do that than to spend money on cutting and threading a barrel and then finding out I should have went an inch shorter.

If the gas (pressure) isn't what fires the bullet through what does?

 

[U.S. Patriot]

An AR does not need that much gas to cycle the bolt. It's not like you are going to loose a considerable amount of pressure between a semi-auto rifle and a bolt action rifle. Meaning, you are not going to have a significant drop in velocity. It's going to depend more on twist rate.

 

[NomadS]

An AR does not need that much gas to cycle the bolt. It's not like you are going to loose a considerable amount of pressure between a semi-auto rifle and a bolt action rifle. Meaning, you are not going to have a significant drop in velocity. It's going to depend more on twist rate.

How much pressure does it need to cycle the bolt? I really have no clue. I have seen many videos where subsonic loads do not cycle the bolt and you have to manually cycle the bolt after each shot. (especially with .22LR)

 

[ROLEXrifleman]

haven't yet. Waiting on the atf to approve the can. YHM titanium.

Planning on starting with 11.o grains of Trail boss and a hornady 180 gr round nose bullet. Will work down from there until subsonic. (have to buy/borrow a chronograph)

Won't put the can on until I get consistent, nice round holes on the target.

Any recommendations?

Now is the time to start load developement not after you get the can.

Seems like you got a lot going in this thread, so I'll be to teh point.

1) You should be working up your load, not down. And don't put a single round through yuor can till you verified that the load your using stabalizes. A baffle strike is not something you want, especialy with an unproven load.

2) You can find out now if your AR will cycle sub loads, you should be running them through your gun now! Also, if your loads cycle teh action now they will also when the can is on as it creates more back pressure.

3) POI will differ between both loads due to teh twist rate. So dont expect same POI with both guns.

4) again, if you load develope now with a load in teh 1000 FPS realm, I gurantee you that if you cut the bolt gun barrel down to the same lenth as teh AR you will remail subsonic.

5) Trailboss is a good powder for bullets between 175 and 208, trial and error. But you need to ERROR without the can on!!!! make sure your not keyholing at 25, 50, 75 and 100 yards.


Load developement for a can is pain staking. Even more so if your trying to develope one load for 2 seperate guns. And now your multiplying that mess with making one gun a bolt gun and teh other a gasser. Not impossible but time consuming.

Did I mention baffle strikes?

The truth is if you want both to be tack drivers in teh subsonic world your gonna end up with two diffrent loads

 

[NomadS]

Now is the time to start load development not after you get the can.

Seems like you got a lot going in this thread, so I'll be to the point.

1) You should be working up your load, not down. And don't put a single round through your can till you verified that the load your using stabilizes. A baffle strike is not something you want, especially with an unproven load.

work up? that sounds like a good way to get a bullet stuck in the barrel. Don't I want to start with a load that is supersonic and then decrease until I get it subsonic? Start with more powder and slowly decrease it until I am subsonic.

2) You can find out now if your AR will cycle sub loads, you should be running them through your gun now! Also, if your loads cycle the action now they will also when the can is on as it creates more back pressure.

Kel Tec RFB actually. It has an adjustable gas system and I have found several threads on other websites where there is no problem. I didn't bring this up originally because I didn't want to get on a side discussion. Main concerned here is wanting to find out how much difference, if any, the bolt action made in fps.. so I can plan where to cut it.

3) POI will differ between both loads due to the twist rate. So don't expect same POI with both guns.

I figured that would probably be the case. Hoping if I can get the fps close to the same though it will minimize the drop difference at least. Doubt I will find one load that produces the smallest group on both weapons.

4) again, if you load develop now with a load in the 1000 FPS realm, I guarantee you that if you cut the bolt gun barrel down to the same length as the AR you will remain subsonic.

So it sounds like the consensus is that the gas system purging will have a negligible effect on FPS when compared to a bolt action with the same barrel length. That actually surprises me. I would have thought the purging of the gas to cycle the bolt would have had a measurable effect on the bullet speed.

5) Trailboss is a good powder for bullets between 175 and 208, trial and error. But you need to ERROR without the can on!!!! make sure your not key-holing at 25, 50, 75 and 100 yards.

yes.. way to expensive to destroy the can and possibly the gun and myself. 180 is what I have now. Kel-tec recommends no bullets over 180 for the setup.

Load developement for a can is pain staking. Even more so if your trying to develop one load for 2 separate guns. And now your multiplying that mess with making one gun a bolt gun and the other a gasser. Not impossible but time consuming.

Yup.. that's why I am trying to pick everyone's brain before I start.

Did I mention baffle strikes?

yup.. and that will be going through my head up until the time I feel comfortable enough to put the can on and pull the trigger.

The truth is if you want both to be tack drivers in the subsonic world your gonna end up with two different loads

understood. And I will probably do that eventually. But right now I want to work up the best compromise I can while favoring the smaller group size to the bolt action.

Thanks for the info.

 

[NomadS]

found this velocity loss semi auto vs.bolt gun - THR They seem to think there is some difference (bolt faster) but the difference is very small. There is probably more variability in fps due to variability in each round and variability in the guns (chambers/barrels) themselves.