Please forgive me if this has been asked before. If you have two rifles with identical barrel lengths, shooting identical ammunition, the only thing different being that one is a semi auto and one is a bolt action. How much velocity loss would there be in the semi-auto? Assume the bolt action has a muzzel velocity of 3000 fps. The caliber for both is .270W. can anyone provide a link to such an answer?
velocity loss semi auto vs.bolt gun
- Thread starter don
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ny32182
Member
In any sort of real world application, there are probably a whole lot more variables other than just the action type that could not be controlled if you are talking about two separate barrels, but purely academically, I would say it depends on what kind of semiauto action you are talking about. I believe a gas operated rifle will give up a fairly marginal amount of velocity by diverting gas to operate the action.
I think someone on FAL files chronoed their rifle with the gas on vs. the gas off (.308 rather than .270 in this case) and measured average veolcity loss of around 30 fps with the gas on, if I remember correctly.
I think someone on FAL files chronoed their rifle with the gas on vs. the gas off (.308 rather than .270 in this case) and measured average veolcity loss of around 30 fps with the gas on, if I remember correctly.
doctorxring
Member
Insignificant loss. On most semi-auto rifles the gas is
tapped towards the muzzle end of the barrel, past
peak pressure point and the bullet's acceleration is well
under way. I would not consider it a factor of choice
between platforms.
dxr
tapped towards the muzzle end of the barrel, past
peak pressure point and the bullet's acceleration is well
under way. I would not consider it a factor of choice
between platforms.
dxr
Dr. Tad Hussein Winslow
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With the same ammo, there is some loss, but I agree it's very insignificant, since the breech is locked almost all the way, and even after the breach is unlocked, no burning powder escapes until the case is fully extracted, and by that time I'd imagine the bullet has cleared those last few inches of barrel., or just almost.
The true "velocity loss" occurs from the fact that you cannot use the hot loads in a semi-auto like the Federal and Hornady light magnums, and hot handloads.
The true "velocity loss" occurs from the fact that you cannot use the hot loads in a semi-auto like the Federal and Hornady light magnums, and hot handloads.
NY32182, that is exactly what I sought, thanks to all for the rapid response. I figured that from a M1 Garand there would be insignificant loss due to the gas port being located near the end of the barrel, but I was unsure about others like a BAR which I intend to get.
rangerruck
Member
doctor and premium are both down here; the loss, depending on powder, primer, and bolt unlock time, is just too insignif to worry about, It may is proly not even 30 fps, because as stated, the pressure has to have dropped enough, just to unlock the bolt, and start moving it rearward, i would guess that a bullet is 1 inch or less from the end of the muzzle ,when this starts to happen. mostly you read the bullet has actually cleared the muzzle, before it starts moving back.
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USSR
Member
don,
Impossible to say, as there are other variables that come into play here. Example: I have two .30-06 bolt action match rifles, both with 26" barrels. With the exact same load, one delivers 2950fps, while the other one only does 2875fps. You simply cannot take an arbitrary velocity from one rifle and subtract/add to it to imply a specific velocity will be reached in another rifle.
Don
Impossible to say, as there are other variables that come into play here. Example: I have two .30-06 bolt action match rifles, both with 26" barrels. With the exact same load, one delivers 2950fps, while the other one only does 2875fps. You simply cannot take an arbitrary velocity from one rifle and subtract/add to it to imply a specific velocity will be reached in another rifle.
Don
MaterDei
Member
I would say it depends on the action type. If we're talking about a blowback design I don't see how you would have any loss. Gas operated rifles would have some drop off I'm sure.
ny32182
Member
The bullet is long gone and pressures have dropped way off by the time the bolt unlocks. If there was still a bullet in the barrel, you have a KB explosion coming back into the action upon unlocking.
Any velocity loss is coming strictly from gas being diverted off to operate the action and/or other uncontrollable factors.
Any velocity loss is coming strictly from gas being diverted off to operate the action and/or other uncontrollable factors.
Don- I posted this question a while back and a first thought you had resurrected my post. I got greatresponses. The short answer is YES - semi auto velocities are less by a measurable amount. Key word measurable. I actually got help from someone who decided to chronograph samples at the range and test the differences scientifically. The end results showed the difference between semi auto and bolt was within the deviations of a single load. In other words the average semi auto velocity was not less than the lowest velocities recorded for a small aggregate of shots out of a bolt action. The highest recorded velocities came from the bolt action and the lowest came from the semi auto but the speeds varied enough to overlap - that is how slight the differences were.
USSR
Member
kenjs1,
So what was the sample size? I don't mean the number of shots fired, but rather the number of rifles that were tested to make the data meaningful.
Don
PercyShelley
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I recall some tests on arfcom using an adjustable gas regulator on an AR-15 that pegged the velocity loss at about 23 FPS, or something in that neighborhood. Just barely statistically significant.
TexasRifleman
Moderator Emeritus
I remember seeing something similar on FAL Files using a FAL and all it's gas settings and the changes from setting to setting were negligible.
rcmodel
Member in memoriam
There is simply no way to test this in two different rifles, out of two different barrels.
The only way to test it and get meaningful results, is with a semi-auto military rifle that has a gas shutoff valve so velocity can be measured in the same exact barrel.
There can and is more velocity variation between two identical barrels from the same maker then there is from two different action types.
At any rate, the loss would be less then the extreme spread in a box of factory ammo.
It certainly isn't enough to make any difference, or enough to worry about.
rcmodel
I did what Navy Guns suggested a few years ago. It was only 10 rounds of each, which isn't a huge sample, but here are the numbers anyway.
Yugo SKS
Wolf 122 HP, green laquer
Distance - 10 ft
ProChrono Digital
Port Open Port Closed
Avg. 2422.2 2442.5
Max 2469 2468
Min 2355 2389
SD 38.4 29.0
ES 114 80
Like others have suggested, it's measurable, but not overly significant in this small sample. Your difference is less than the SD.
Eventually I plan to do a similar test with a buddy's M1A, but I've said that for 2 years now.
Yugo SKS
Wolf 122 HP, green laquer
Distance - 10 ft
ProChrono Digital
Port Open Port Closed
Avg. 2422.2 2442.5
Max 2469 2468
Min 2355 2389
SD 38.4 29.0
ES 114 80
Like others have suggested, it's measurable, but not overly significant in this small sample. Your difference is less than the SD.
Eventually I plan to do a similar test with a buddy's M1A, but I've said that for 2 years now.
rcmodel
Member in memoriam
Great info g1726!
Thanks!
rcmodel
Thanks!
rcmodel
I try. That was one of those "gee-whiz" kind of tests that I did because I could. 10 shots isn't great for a test like this, but it confirmed what others have said with real data.
When I got my chronograph about 3 years ago, I created a spreadsheet that does all the calculations for me to include recoil energy, recoil velocity, etc. I've kept it up pretty good, so I have all my data along with date, time, temp, gun, barrel length, etc. It's nice to make comparisons. I have 56 samples for .22 LR, which is somewhat interesting when you look at recoil impulse and why some guns function better or only with 40 gr. vs. 36 gr. bulk pack stuff.
I know two other guys with Yugo's, so someday I might do a bit more statistically significant test. But someday hasn't gotten here yet!
When I got my chronograph about 3 years ago, I created a spreadsheet that does all the calculations for me to include recoil energy, recoil velocity, etc. I've kept it up pretty good, so I have all my data along with date, time, temp, gun, barrel length, etc. It's nice to make comparisons. I have 56 samples for .22 LR, which is somewhat interesting when you look at recoil impulse and why some guns function better or only with 40 gr. vs. 36 gr. bulk pack stuff.
I know two other guys with Yugo's, so someday I might do a bit more statistically significant test. But someday hasn't gotten here yet!
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KBintheSLC
Member
I believe this is true. A blowback action does not even begin to move from locked breech until after the bullet leaves the bore. A piston operated action begins to divert a portion of the expanding gases before the bullet leaves the bore... which will likely result in a negligible loss of velocity.
I believe this is true. A blowback action does not even begin to move from locked breech until after the bullet leaves the bore. A piston operated action begins to divert a portion of the expanding gases before the bullet leaves the bore... which will likely result in a negligible loss of velocity.
__________________
Dirka Dirka.
DD I am not trying to start an arguement for doubtlessly you are better versed than I on this, but a friend of mine noticed fire coming from the action of my Marlin model 60 ( blow back action) when I fired it. Wouldn't that reduce the velocity because the powder was still burning?
Thanks again to all respondents. You have answered my question.
__________________
Dirka Dirka.
DD I am not trying to start an arguement for doubtlessly you are better versed than I on this, but a friend of mine noticed fire coming from the action of my Marlin model 60 ( blow back action) when I fired it. Wouldn't that reduce the velocity because the powder was still burning?
Thanks again to all respondents. You have answered my question.
Z-Michigan
Member
All the big stuff has been summed up, but here's a few details IMHO:
-M1 Garand is about the ideal no-loss platform since the gas port is only inches from the muzzle. Incidentally, as Garand owners know, running ammo with really slow burning powder can cause a bent op rod in that particular design, since it was intended for cartridges with moderate speed powder, that has relatively little residual pressure at the muzzle.
-I would think a 16" AR-15 with a carbine length gas system would be the opposite extreme, since the gas port is so close to the breech that the gas is at really high pressure, and so far from the muzzle that there's some real bullet flight time between passing the gas port and exiting the barrel.
-Old tests on the Garand showed the bullet was something like 20 feet out of the muzzle before any action parts were moving measurably. I expect this is true on most designs, with the exception of AR's having short gas systems and relatively long barrels (again, that 16" barrel with a carbine gas system - a combination that the designer never intended). But even for those I'm sure the bullet is fully out of the barrel before anything unlocks - otherwise cases would kaboom.
-Though I don't have any data, I will bet that simple blowback designs actually show the greatest velocity loss vs. a locked breech, since the case will start moving rearward instantly and the bore diameter x say a half inch of movement creates a much larger increase in volume for gas to move into than a tiny 0.05" gas port into a .25" gas piston or 0.15" (guess) AR gas tube.
-Don't forget that recoil operated firearms can also see a loss, since the barrel will have started moving backward to some degree by the time the bullet leaves. I'll bet the loss is tiny for most of them, but I'd be interested in any actual data. The only easy way I see to get that data would be if you have one of those special ops type pistols that will let you lock the slide in place for use with a suppressor.
-M1 Garand is about the ideal no-loss platform since the gas port is only inches from the muzzle. Incidentally, as Garand owners know, running ammo with really slow burning powder can cause a bent op rod in that particular design, since it was intended for cartridges with moderate speed powder, that has relatively little residual pressure at the muzzle.
-I would think a 16" AR-15 with a carbine length gas system would be the opposite extreme, since the gas port is so close to the breech that the gas is at really high pressure, and so far from the muzzle that there's some real bullet flight time between passing the gas port and exiting the barrel.
-Old tests on the Garand showed the bullet was something like 20 feet out of the muzzle before any action parts were moving measurably. I expect this is true on most designs, with the exception of AR's having short gas systems and relatively long barrels (again, that 16" barrel with a carbine gas system - a combination that the designer never intended). But even for those I'm sure the bullet is fully out of the barrel before anything unlocks - otherwise cases would kaboom.
-Though I don't have any data, I will bet that simple blowback designs actually show the greatest velocity loss vs. a locked breech, since the case will start moving rearward instantly and the bore diameter x say a half inch of movement creates a much larger increase in volume for gas to move into than a tiny 0.05" gas port into a .25" gas piston or 0.15" (guess) AR gas tube.
-Don't forget that recoil operated firearms can also see a loss, since the barrel will have started moving backward to some degree by the time the bullet leaves. I'll bet the loss is tiny for most of them, but I'd be interested in any actual data. The only easy way I see to get that data would be if you have one of those special ops type pistols that will let you lock the slide in place for use with a suppressor.
Wildfire
Member
+1 Z_Michigan.
Hey there;
I was wondering if any one was gonna get to that point.
Hot loads in an M1- Garand will bend OP-Rods.
Hey there;
I was wondering if any one was gonna get to that point.
Hot loads in an M1- Garand will bend OP-Rods.
Are blow back systems the same as recoil systems? I had an old remington shotgun, semi auto and the barrel moved to the rear when fired, yet my marlin model 60 barrel does not. The Remington was built on a Browning patent as I recall. Which is blow back and which is recoil operated? Z-Michigan if you think the gas tube is close on the AR-15 or M-16 carbine you should see the Wildey pistol.
Z-Michigan
Member
Blowback and recoil are completely different systems. Unfortunately some people do confuse them (lately, CDNN's catalog entry on the PTR91 is a notable offender).
Blowback systems are common for .22's and some pistol caliber weapons (e.g. Uzi, MAC-10). The bolt is literally blown back by the pressure of the gases pushing on the case. Usually the bolt has to be very heavy to slow opening enough to be safe. These systems are very simple, but reliability varies with cleaning and other factors.
There are also delayed blowback systems like those used in the CETME and various HK guns. These are more complicated but are still blowback, not recoil operated.
Recoil operation means the barrel and bolt both move backward relative to the frame for at least a short distance, after which the barrel is stopped and the bolt isn't, opening the breech and operating the whole cycle. This is the dominant system for centerfire pistols (thank Browning!). It has also been fairly common in larger machine guns (like the MG34, MG42, and M-2 Browning .50). It is rare in conventional rifles and shotguns, though not unheard of - the Browning Auto-5 shotgun is one example.
To make things even more complicated, Benelli has their inertia operated system that is effectively a form of recoil operation, but works completely different from normal recoil mechanisms. But I digress.
Blowback systems are common for .22's and some pistol caliber weapons (e.g. Uzi, MAC-10). The bolt is literally blown back by the pressure of the gases pushing on the case. Usually the bolt has to be very heavy to slow opening enough to be safe. These systems are very simple, but reliability varies with cleaning and other factors.
There are also delayed blowback systems like those used in the CETME and various HK guns. These are more complicated but are still blowback, not recoil operated.
Recoil operation means the barrel and bolt both move backward relative to the frame for at least a short distance, after which the barrel is stopped and the bolt isn't, opening the breech and operating the whole cycle. This is the dominant system for centerfire pistols (thank Browning!). It has also been fairly common in larger machine guns (like the MG34, MG42, and M-2 Browning .50). It is rare in conventional rifles and shotguns, though not unheard of - the Browning Auto-5 shotgun is one example.
To make things even more complicated, Benelli has their inertia operated system that is effectively a form of recoil operation, but works completely different from normal recoil mechanisms. But I digress.
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