I have a Stage model 8 + with a Nikon 223 3x12 scope.  It's gas on or gas off no other adjustment. The other day I tried shooting about 200 yards semi-auto then turned gas off for single shots. I'm not a perfict shot but the single shots had a tighter group. What is going on. Is it acting like a bolt action. Without use of the piston the bolt would still move from pressure? Could lack of action cycling make a difference? But the bullet leaves the barrel before the action cycles?

With the gas off, the bolt isn't going to move, because chamber pressure is not what cycles the action (otherwise you'd have a kaboom EVERY round). What makes the action cycle is gas bleeding off through the gas port acting on the bolt carrier (either via a piston and op rod or gas tube/DI mechanism) to unlock the bolt. If you cut off that gas, the bolt carrier will not move, and the bolt will not unlock.

The normal action cycle starts while the bullet is still in the barrel, as soon as it passes the gas port. It doesn't complete until after the bullet has left the barrel, but it does start while the bullet is still in the barrel. So it is possible for normal cycling to cause vibrations in the barrel while the bullet is still inside that could degrade accuracy to some degree. That is why most high-end sniper rifles are bolt action instead of semi-auto. Under normal circumstances it's not a significant factor, but if you're trying to hit a mosquito in the ass at 600 yards, every little bit helps.

You are correct, projo is long gone by the time the action starts cycling, no matter the operation type.  

What you have not mentioned are the other variables, mainly being the ammo used.  If you're shooting ball ammo (i.e. M193) the accuracy requirements were in the neighborhood of 3moa.  This means that with the greatest benchrest AR on the planet you would be doing well to go under 2moa.  The requirement is linked to the QC of the ammo though.  As the manufacturer could have that much variability in both the bullet production and the powder charge wringing out great accuracy with less-than-great ammo is nearly impossible.

If I shoot standard ball (55gr M193) in my 1:8 Rainier Select barrel I typically go a bit over 2" at 100yds.  When I use the ammo I've worked up for it (SMKs over Varget) that shrinks to a 3/4" average, with the gas on in my AA system.

Your rifle will be specific as to what works best with it.  If you reload build your load ladder with a quality projectile that matches your twist rate well (i.e. those SMKs) and find what it likes best.  Once you do that then and only then do your groups matter.  If you don't reload find some of the Mk 262 as this is a match-quality load meant for 1:8 or 1:7 barrels.  I bet you will find that everything is identical, gas or no.

 
So all bolt action sniper rifles are now hereby surplanted by semi auto platforms? Thanks for enlightening us. As soon as word of your post goes viral, it will be a matter of days before bolt action manufacturers cease production. Again, thank you for your wisdom on this subject.  


 

Can I just say that rifle action isn't the only and final reason sniper's might choose a bolt action?  You need nothing more than to look back at the transition from .30-06 to .308 in the US military to see one of the major delineations between semi-auto and bolt.  Long action rifle rounds are not well suited to full-auto actions.  Timing the action to be long enough to cycle the rounds becomes increasingly difficult as the cartridge OAL becomes longer.  So the advent of the .308 which was intended to approximate the .30-06 but with a shorter OAL.  The trade off is final bullet speed.  Faster bullets fly flatter, so longer cases mean less adjustment for elevations.  But a fully auto or even semi-auto doesn't do well with longer cases.  It's really hard to make a reliable .300 WinMag in full auto so you're not likely to see a lot of them made.  Which means the best choice for .300 winmag is bolt.  Similar case for even the .338 Lapua.  But there are semi-auto "sniper" rifles.  Look at the M110 or any of the other .308 (short action) semi-auto sniper rifles.

Leaving it all down to the action alone is WAY too oversimplified.

Let's talk physics.  We all know the term dwell time.  On the typical AR style rifle using a carbine gas system and a 16" barrel, the dwell time of the bullet after the gas port is around 0.00025 seconds.  That's assuming a 3000 fps bullet and 9 inches of dwell in the barrel.  0.00025 seconds.  That is not enough time to actually start the cycle.  In fact, that's barely enough time for the gas to have made the trip through the port and back down the gas tube to the BCG.  By the time the bullet has left the barrel, the gas is just barely starting to expand the gas chamber in the BCG and push the bolt face into the barrel chamber.  The action has not even started moving yet.  So no, the action doesn't start moving before the bullet leaves the end of the barrel.  On a piston style AR, the gas is able to start the action sooner, but 0.00025 seconds is not enough to actually get that mass moving.  It takes at least that just to fill the piston cup.

There are a LOT of reasons why a person might choose bolt action over semi-auto, but the myth that bolt actions are inherently more accurate is not really one of them.  They are inherently less complicated, which for some people is reason enough.  If you're only firing one shot, what benefit do you gain from the complex action of a semi-auto?

With a rifle length DI gas system,There is a 0.2 millisecond (0.0002 second) delay between the bullet passing the gas port to the first pressure rise in the bolt carrier cavity (the time it takes the gas pressure wave to travel the length of the tube), peak pressure in the cavity occurs about 0.8 ms later.  The delay for a carbine would be about half that, if the gas port was the same size, since the tube is half the length of a rifle system, but the gas port is usually smaller for the carbine system, so the delay is about the same 0.2 ms.

Obviously, as soon as the pressure rises above ambient in the bolt carrier cavity, the bolt carrier starts to move, but during the first 0.5 ms the bolt carrier has only moved about 0.002 inch.  By the time peak pressure in the bolt carrier cavity has occurs, the bolt carrier has moved all of 0.010".

Unlocking starts about 1.5 ms after the bullet has passed the gas port, or about 1.0 ms after the bullet has left the barrel.  Unlocking is complete 2.5 ms after the bullet has passed the gas port, or 2.0 ms after the bullet has left the barrel.

If you look at the bolt carrier distance, velocity and cavity pressure timing, you may note that most all of this occurs after the bullet has left the barrel, or what is called 'blow-down'.  

For a piston system like the Adams Arms, there is still a 0.0002 second delay between the bullet passing the port to the bolt carrier moving, due to the gap between the piston and carrier.  There are other tricks used by various manufacturers to keep the bolt acceleration slow at first, then allow greater acceleration, even though the pressure is dropping.  All the other timing is similar.