Home Forums Hunting & Shooting Ask The Gunwriters does a semi auto action decrease bullet speed?

I say no - or practically nill but I have a very stubborn technically minded friend who wants a good explanation for if\why a bullet shot out of a semi auto wouldn't be going slower than one out of a bolt? His belief, that I have failed to counter to satisfaction, is that the energy used to cycle the action must be reducing the "accelleration energy" available to the bullet itself? Someone help me with this one please. Feel free to be as mathematical as you like. I was finally able to convince him that muzzle breaks work but that took some doing. Just this weekend I gave him examples on recoil that he was OK with but never got this particular item put to rest.

IMHO the gas used to cycle the action will decrease the speed as compared to a bolt gun. I may be wrong, but I have never chrono'ed a semo auto that was as fast as the same ammo was in a bolt gun with the same barrel lenght..The difference IME was between 100 and 200 fps difference...

In theory, yes. In practice, no.

First the gas "lost" isn't really lost completely. So even though there's a small pressure drop, it is very VERY small, percentage-wise.

Second, most gas actions tap the gas fairly near the muzzle, so there's little time left before the bullet clears the muzzle - and nearly all the bullet speed already attained.

Third, you'd have a hard time deciding, because the individual variation from shot to shot is almost always larger than the loss of speed due to gas bleedoff.

All these can be clearly demonstrated by simply blocking or turning off the gas system (in guns like the Garand where you can do so.) Measure bullet speed with the gas system on and off and compare.

I sure have to agree with Rocky on this account. In fact, I had a Remington 742 in 243 Win and a Win Model 70 in 243. With identical loads the 742 shot faster over my chrono. Maybe the Remington just had a "faster" barrel than the Win.
So will a gas auto shoot slower? ,, maybe.. but sure not enough to make a bit of difference downrange. Mine actually shot faster,, go figure??

BINGO--Apples to apples is the only way to measure for sure.

BMT

Years ago in Precision Shooting magazine there was an article concerning how many FPS were lost from gas bleed off.
The Author I believe was Derrick Martin, and he took a barrel off an AR15 and mounted it on another action.
He left the gas hole open and did tests over a cronograph.If I remember correctly it was about 7 fps.

Ask your friend this rhetorical question as a way of explaining what happens.

"Does the water level in the ocean rise if you pee in it?"

"Does the water level in the ocean rise if you pee in it?" [/quote]

Yes it does, just about the same as the difference between a semi auto and a bolt action,,, not enough to get excited about or notice.

Rocky (and others), after receiving the the first two responses I cut and posted them in a note and added comments that turned out to be nearly identical to yours. Being unfamiliar with firearms workings I think his mental assumption about recoil (identified by my last comments to him)is likened to a recoil operated rather than gas cycling action. My take is that if talking about a recoil operated action then the best answer might be that it takes very little energy for the bullet to break the bond of a crimped case that holds it - though I might never believed that after hammering away on one of those blasted enertia bullet pullers. Here are my responses if interested. Let me know how to improve on them - or if I am in error. Thanks folks.

Pete,
thought I would ask your question about speed loss due to cycling a semi automatic action to see if anyone came up with something different. Here are the first two responses:



I call this a "possible indication" more than scientifically measured. The reason might be that this particular barrel is slower than others. Two identical loads fired from two different barrels, even if the barrels are on identical rifle models, will yield at least slightly different speeds. Other factors in barrel speed include manufacturing techniques- ie button rifled or hammer forged as well as how fresh the cutting tools were when they made a particular barrel. If I bought a box of 270 ammo and put one round in my CZ and one in a Savage there would be a difference in speed. Just the nature of absolutes. Certain makes do 'tend' to be faster than others and twist rate of the rifling is assumed to be the same here- BUT in certain calibers you can find different twist rates because some are geared towards shooting lighter or heavier bullets in that caliber. 223 is a good example of this. So, overall is there a difference? A small one possibly is the answer- just as suspected.
Here is the second response:



Note here Pete is that I believe you were looking at recoil in the way it is used in an inertia system firearm. Like a Benelli rifle or shotgun or a lot of semi pistols. The recoil on these are noticeably sharper in my experience- but the cycle times are faster. You can shoot a Benelli recoil\enertia operated semiauto shotgun - maybe some Beretta's too - Martin's new one might be recoil operated, -anyway you can pop three rounds off with amazing speed. Very fun but you don't want to keep doing it. For a practical application of your question as it pertains to rifles we should consider it as it pertains to gas operated devices and that is what the post above is speaking to.

Your friend may have in mind a simi-auto "blow-back" system, which most if not all simi-auto .22`s use. In my experience, and non-scientific testing, he`s right.

Still no loss. In recoil-operated and "blowback" systems, the action does not begin to open until the pressure in the barrel drops almost to zero - meaning the bullet has left the muzzle. That being the case, there can be no possible effect on velocity.

Rocky's right-
Nothing happens, action-wise, until Elvis has left the building.

I have all the respect in the world for Rocky's expertise. That being said, I have compared Browning BAR's in 25-06 and 280 to bolt guns by shooting the same the loads side by side over a chrony.

Chrony said the BAR was shooting the same loads ~ 200 fps slower.

I believe Browning has publicly admitted the same.

Of course the 200 fps only matters to the chrony. Neither the operator nor the intended critter will notice a practical difference without reading the chrony.

GE

differant barrels could account for that much ..

That's true. Two bolt guns an be that far apart. The real test would be if you could block the gas port in the BAR and shoot it that way. I bet there'd be absolutely no notable difference in the muzzle zoom.

I have a Ruger Mini-30 with an adjustable gas block. This allows me to adjust the size of the gas port. I will load 20 identical rounds. Fire 10 with the gas port open and 10 with the gas port completely shut off and record the velocities. I will try and get this done on Sat. Maybe this will help settle this.

hey thanks Steve- it is kind of interesting and something a lot of us have probably wondered. Looking forward to the results. anyone taking bets???

I will bet one BoreSnake (new) in 12-gauge size that there is no appreciable difference.

.

This sounds very interesting. I am suffering velocity loss because of the energy required to cycle the action? Then how much energy/velocity loss is caused by recoil itself, even in a bolt gun? A heavy recoiling rifle or shotgun, bolt or semi-auto, expends a lot of energy in coming back to thump the shooter. How much velocity does that cost?

So, if I place the butt of my 30-06 against a tree to prevent recoil motion I should see a velocity gain when fired, as compared to firing from my sholder? Hmmm...

How much velocity do I gain when shooting road signs and mailboxes from my truck traveling 60 mph?

This is giving me a headache.

.

About 88 fps subject for adjustment between angle of travel and sign. Which is also subject to whether the gunner is A) Driving B) Drinking C) Passenger D)A&B or E) B&C.

Questions like this on the SAT would fill Harvard with kids from North Dakota.

GE