shot string question

[elmury]

Hello all,

I am in the process of testing my BSA Buccaneer .22 and noticed something I find strange.

I fill the gun up to 200 bar and chrony some shot strings at an indoor target at 10m.
Muzzle velocity starts at ~225 m/s, goes up to ~238 m/s and goes back to ~220m/s at shot 40 at ~80 bar.

The POI stays the same for the first part of the shot strings but starts to climb ~8mm at the part where the velocity decreases back to 220m/s.

So my question is, how can it be that the POI has shift while the muzzle velocity is the same?

(Sorry for the metric values. Will change them if needed)

[Booger]

What you are describing is called a shot curve. It can be eliminated with a regulator.

I moved your post to the PCP gate, so you can get more answers to your question.

[elmury]

I moved your post to the PCP gate, so you can get more answers to your question.

Thank you and sorry for posting in the wrong section.

What you are describing is called a shot curve. It can be eliminated with a regulator.

Yes, I understand that a regulator would keep the pressure per shot constant.

Still, on all shot curve pictures I've seen the POI goes down at the end of the unregulated shot string.
How can mine go up 8mm @10m distance while the velocity was the same.
Has this to do with the different pressure build-up behind the pellet?
I.e. a short burst @200 bar versus long burst @80 bar gives same velocity but different POI?

[RDB]

Newbie here...but been tinkering and reading all that I can from the guru's here for about five years now.

I think your on the right track with your thinking. Towards the middle and end of your string you have more valve dwell and you may be belching more air out behind the pellet after it has left the barrel causing poi shift.

I would try a heavier pellet or back the power down and see if that straightens it out.

If I'm out in left field on my thinking here one of the more knowledgeable guru's will be along shortly to help you out.

[elmury]

...Towards the middle and end of your string you have more valve dwell and you may be belching more air out behind the pellet after it has left the barrel causing poi shift.

Hi, thanks for your reply. Appreciate it.
Yes, i thought of that too but it's my understanding that once a pellet has left the barrel it doesn't get propelled any more and all it does is fall.
I could imagine it being blown out of course by the 'after belching', ... but pushing it upwards? Dunno.
The measured muzzle velocity was the same, remember.

Maybe its about increased lock time.
The gun feels quite different whilst shooting @200 bar or @80 bar.

Or it could be mechanical. At 200 bar the tube may expand a little more tilting the barrel ever so slightly then at 80 bar.
Hmm, I'm really puzzled by this
Just thinking out loud.

[rsterne]

My bet is mechanical bending of the tube when the pressure changes, influencing the barrel.... If not that, then the differing pressure pulse is changing the barrel harmonics and causing the POI shift.... At the beginning of the shot string, the pellet is getting a short blast of very high pressure.... at the end of the string, even though the velocity is the same, it is getting a longer, slower push of lower pressure air.... more of a "shove" instead of being "hammered"....

Bob

[elmury]

My bet is mechanical bending of the tube when the pressure changes, influencing the barrel.... If not that, then the differing pressure pulse is changing the barrel harmonics and causing the POI shift.... At the beginning of the shot string, the pellet is getting a short blast of very high pressure.... at the end of the string, even though the velocity is the same, it is getting a longer, slower push of lower pressure air.... more of a "shove" instead of being "hammered"....

Bob

Hi Bob,

Thanks a lot for the explanation.
I this a common thing to witness in pcp airguns or is it a design flaw/shortcoming of the BSA Buccaneer or just the one I got?

I must say that i find the mounting method of the breech block to the tube quite flimsy.
Just 2 m3 bolts at a weak point of the barrel receiver. And 1 m3 bolt at the very end of the block.
If i'd remove the barrel band i fear that a bump against the muzzle would bend the front part of the breech.
Or am I worrying to much?