Muzzle velocity Differences between bolt release and cycled round??

[wrinklestar]

I am seeing on average a 35 fps increase on any rounds that have cycled into the chamber. The first round when I hit the bolt release is consistently around 2500 fps. the subsequent rounds are all 2535 ish. Is this consistent with what some of you are seeing?
The gun is a Vltor upper, shillen barrel, non adjustable gas block, std spring and buffer with an m16 (full mass) bolt carrier.
The reason for the question is I am ordering a quigley -ford scope I purchased at an auction a couple of years ago and the fps discrepency may prove to be significant at longer range.
Thanks

[grayfox]

1 round vs a second is not the way to find or settle on MV. Need to run some groups, 4-5 rounds, several times and compute averages as well as SD's, then you can work with what it's shooting whether long or short range.
Your avg could be 2517-ish so 18 high, 17 low wouldn't deviate that much at distance... esp since you don't really know what the MV will be for a given round... that's where avg and SD come into play to give you details you can use.

Personally I like SD's in the single digits, ES in the teens, but elsewhere there is a good discussion that even a 20-ish SD can shoot accurately at distance... this confirmed by Klem who is a competition LR shooter...

[wrinklestar]

I have shot many rounds this last weekend
10 using the bolt release SD 11.9 ES 39 avg MV 2501
10 after the action cycled via GAS ...SD 9.6 ES 27 avg MV 2536

My question is really it seems to do this with all factory loads and reloads. The fastest of the bolt release rounds never makes to the bottom end of the slowest gas cycled rounds. The cycled rounds are ALWAYS 35ish fps faster. Is this common or am I the only one seeing this?

[Keef]

Not sure if MV is the reason, but I have read many AR15 threads about the first, hand charged, round having a different poa/poi than gas fed rounds.

[NugginFutz]

Welcome to the Horde, wrinklestar.

Next time you're at the range, do the following.

Measure the COAL of a round and chamber it using the bolt release, but do not fire it. Eject that round and observe whether or not there is any resistance to extraction, marks on the bullet, etc. Re-measure to see if it is any longer.

Now, measure another round, and place it in the magazine, beneath another. Insert the magazine and chamber the first round. Fire that round, but not the second. Extract the cycled round, and repeat the above steps. If there is any indication that the round was jammed forward into the lands or was sticking into the chamber, in any way, you likely will have your answer. (Higher pressure.)

Hey, it's just a theory, right?

[Sticks]

...
Hey, it's just a theory, right?

Not theory. Good test, and he will find bullet jump, especially if he has a really strong recoil spring or really weak, and smooth upper. Bolt bounce will cause an increase in bolt velocity on the way back.

[wrinklestar]

Thank You Gentlemen!
I will try that the next time I go to the range and see how it goes.
It is a nickel Boron Bolt from WMD, shillen Bolt, Vltor upper.
Thank you for the welcome!

[Sinclair]

The name of the game is the same. A bolt released from locked open position does not move forward with the same force as A bolt that has recoiled and hit the "end" of the buffer. Hence the cartridge is jammed into the camber with a different force creating a difference in the amount of whatever of the case during firing. This phenomenon is why auto loaders do not normally display the same level of accuracy as a bolt gun can. Look at the US military's search for a perfect sniper rifle. Trying it get past 800 yards has been a challenge while many of the contemporary bolt guns used by other Nations can reach out to 1200 yards.. Back in the day I had a FAL that exhibited the same phenomenon and yes if the range is great enough the first shot will go low..

[Klem]

The slight statistical difference could be explained by inertia.

When the cartridge goes into battery it is doing so with two different velocities. One from whatever energy is stored in the recoil spring from the bolt release point, and a faster velocity from a completely compressed spring when the gun cycles. When the round comes to a sudden stop in the chamber the bullet creeps forward from inertia. Like Nuggin says, if you unload the round without firing it is likely you will see two different OAL's. One from bolt release and one from cycling. The two different OAL's mean there are now different case capacities for the powder to burn and different neck tensions for the bullet to release.

Unless your bullet has a cannelure that you can crimp the neck into (like military rounds) it is almost impossible to eliminate this. With decent neck tension and surface area gripping the shank then you will at least minimise it. One of the reasons you won't see auto-loaders winning any Bench Rest competitions I guess.

(just seen Sinclair's post above me...what he said!)

[LRRPF52]

Funny. Looking back at years of chronograph notes, I see a general pattern with about a 30-35fps increase in most strings as well.

The first round is usually, but not always, about 30-35fps slower than the rest of the string.

Kyle Lamb recommends aggressively charging the AR15 like a sling shot for the first round, from a bolt-closed starting point with a fresh mag.

[Sticks]

One wonders if you tune the gas system to the ammo, to be as soft as possible, that you may eliminate the violence of the chambering during cycling.

JP-SCS with light spring, low mass carrier, Adj GB dialed way down.

[A5BLASTER]

One wonders if you tune the gas system to the ammo, to be as soft as possible, that you may eliminate the violence of the chambering during cycling.

JP-SCS with light spring, low mass carrier, Adj GB dialed way down.

Nope it still does it, I have that exact setup in my 20 and it still happens.

At least my chronagrah still shows that it does.

[kpswihart]

Kyle Lamb recommends aggressively charging the AR15 like a sling shot for the first round, from a bolt-closed starting point with a fresh mag.

I spent time as an enlisted infantryman in the 80's and 90's. I don't remember any training, during my time, that recommended not using the bolt catch for initial charging or reloading. But, during my 20 plus years as an LEO in Alaska, we had it hammered in to us to never use it. Build muscle memory necessary to "slingshot" the bolt and you'll likely perform the same under stress. Running the bolt hard will provide more consistent and reliable feeding and locking. Old habits die hard and I still "slingshot" my bolt. I have not noticed the velocity differences.

[wrinklestar]

Well I finally made it back to the range and the overall length did increase a reliable .001 each time. 2.245 to 2.246 everytime seems not all that significant but looking at the later posts it seems that it may be the nature of the beast.
Thanks

[NugginFutz]

Well I finally made it back to the range and the overall length did increase a reliable .001 each time. 2.245 to 2.246 everytime seems not all that significant but looking at the later posts it seems that it may be the nature of the beast.
Thanks

Glad you now have your answer. At the very least, you now have a valuable data point.

Does this make much difference in a hunting scenario? Probably not. Going out to 800+ yards? Yeah, there will be a bit of difference between Shot #1 and Shots #1+n. This is one of the reasons why so many bench shooters hand feed their AR's, even when the rounds will fit in a magazine. Me? Steel targets don't seem to mind a couple of inches low/high, and I don't yet shoot in competitions, so it's magazines for my shooters.