Other Here's a theory that I've developed. Hoping for confirmation...

The spring rifles definitely "warm up" and this effects the POI.....most of the time. There are always exceptions.

I have a HW97k that always shoots POA/POI tuned and polished @ 11.6 FPE and H&N 8.44 FTT . Also an HW 30 that takes a few to worm up , actually never gave it a thought .

As explained to me and I'm sort of paraphrasing... As you shoot, the piston seal "warms up", lubes 'may' move around/change viscosity and these things change the guns harmonics... The tighter the seal and more lube present, the greater the effects (more or less).

...or something to this effect.

*edited in...
I've read, but cannot confirm (some?) piston seals might actually get firmer when warmed up... But this is beyond my paygrade. As long as they work, I could care less either way

I always take a few warm up shots before I check poi or go hunting.

razor62 said:

So, it would seem be be the case that the first shot will have a different POI than say, shot number twelve. With this in mind, I wonder about how this affects a hunting senario, where the first shot is almost always the one that counts. Armed with an understanding of this phenomenon, if you were to have a piston rifle as a dedicated hunting gun, it might make sense to do the final zeroing process over a period of days so that the rifle is not "warmed up" while dialing it in. Still, colder weather will play a role too so, one may have to re-zero when temps are low once the rifle has equalized with the environments' temperature.
I kinda' do something similar with my whitetail hunting muzzleloaders. POI with a muzzleloader will often times change as the bore begins to "foul". I always clean the bore between shots when zeroing to account for this.

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I knew this was coming...

One example I have is a Remington VTR 725 .25cal, basically a Trail XL in synthetic dress... It's sighted in for a 'cold bore' shot, in fact 3 or 4 of them until the POI changes to more than my liking. It slings 25.4gr JSB's at an average of 708fps with an ES of something like 6 or 8fps or so... Yes, changes in atmosphere will affect POI, but usually not enough within 30 or 40 yards to make a huge difference... At least with a .25cal (kidding, well sorta )

PEP3 said:

I always take a few warm up shots before I check poi or go hunting.

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This is very good practice, especially with pcp's or other guns with... Um, flimsy assembly(?) You know, look at it wrong and the barrel shifts 1mm or better and you can't hit anything... Or if you know your scope got knocked hard.

Don't flame me too hard, I've had very solid/reliable pcp's too. Just usually springers can take quite a bit more 'handling' without worries of your barrel shifting around. YMMV

Just Zack 2 said:

This is very good practice, especially with pcp's or other guns with... Um, flimsy assembly(?) You know, look at it wrong and the barrel shifts 1mm or better and you can't hit anything... Or if you know your scope got knocked hard.

Don't flame me too hard, I've had very solid/reliable pcp's too. Just usually springers can take quite a bit more 'handling' without worries of your barrel shifting around. YMMV

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That's funny, it's the complete opposite for me. I can completely disassemble my pcp's, put them back together and the poi never changes. I take the stock off any of my springers and I have to re zero the scope.

PEP3 said:

That's funny, it's the complete opposite for me. I can completely disassemble my pcp's, put them back together and the poi never changes. I take the stock off any of my springers and I have to re zero the scope.

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I'm envious of you. I spent eight months to a year trying to sort out POI shifts of one of mine... I'll try not to detail this topic too much, sorry. Carry on...

internals limber up a bit and the pellets re-season the bore. Much like 22 rim fire first couple shots are going to be off from the rest in that string. changing type or brand of projectiles will also cause this. Squeaky clean bores always take a few shots to settle in. in competition we never trusted a cold bore.

May God love and protect these “gremlins”. Without them I would have become bored with and lost interest in air gunning long ago. Thankfully accurate quality springers doing battle with the gremlins keeps my interest alive and well. Uj

When I grab a springer I will cycle the piston with the cocking lever several times without fully cocking it.

razor62 said:

So, I've been noticing this on MOST, but not all of my springers. For instance, my .22 cal HW 97K is an exception. What I'm seeing is that after I've fired a dozen or show shots from a piston powered gun, the groups begin to tighten up. Now, I understand that this could just be due to me getting warmed up, but here's my theory. I'll bet that it's not a new idea. This must've crossed many an airgunners' mind....
Could it be that the piston seal is heating up during the first few shots and that once its become more pliable, it forms a more consistent seal? It just seems to make sense to me that this would be the case. However, as I've already mentioned, my 97K is one of the few piston guns that I own which always seems to shoot point of aim no matter how long between shooting sessions, so.....
What are your thoughts...?

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sounds reasonable.

I just finished running my springers over the chronograph. On most there I find a drop in velocity over the first 10-15 shots as the gun warms up. At least I assume it is due to the gun getting warmer. The drop amounts to around 10 fps and is only noticeable on longer targets. As far as groups tightening, I attribute that to me having to relearn the best hold for a particular gun. And sometimes groups will open up as I shoot, which I attribute to fatigue or loss of concentration. Sure keeps it interesting!

Ok there's a few things to consider a piston gun "warm up" period. There's actually alot but I'll try to keep this to the basics.

If a gun needs to cycle a few times to loosen up the lubricant it probably either has too much and or the wrong lubricant. Piston guns need very little lubricant. Unless your shooting in freezing weather lubrication should not make a difference.

Piston seals will change dimensions with heat. They typically become larger, thus tighter and slower. There's heat that comes from air compression. There's a flash of heat in each cycle hot enough to ignite most lubricants. This heat flash only last milliseconds and doesn't last long enough to normally raise the overall gun temps. Any slight amouts of conducted heat is easily radiated away by the metal compression tube and piston under normal conditions.

Still its pretty common for piston guns to settle down a 10 fps in the first few shots. I believe this is more the result of exercising the seal back to full size from air pressure.

However during prolonged use compression heat doesn't completely dissipate as well and the gun will some times slow even more. You'll see POI shift and groups open up. Its because the seal becomes tight to the bore and loose on the piston anvil. The number of shots it takes to see this depend on the rifle, rate of fire and ambient temperature. During cooler weather you may never experience this.

However in hot sunny conditions a black steel tube gets hot enough on its own that it can't dissipate the compression heat well. After extended use you'll be able to feel the compression end of the tube is actually hotter than the rest of the gun. I've experienced >40fps losses and had groups turn to complete garbage. Put the gun in the shade a half hour and it goes back to shooting normally.

Not all piston guns experience this to the same extremes because not all piston guns use the same seal design. The Weihrauchs I love so much are probably the most temperature temperamental because their piston seal is a solid cap that spans the full diameter of the bore. This causes more thermal expansion than other designs. Hollow piston seal designs like some Dianas and AA are far less suseptible to heat issues.