On Saturday, I shot my SamYang 909 in too hot (30+ degrees Celcius), windy and uncomfortable conditions to enable any good 50 yard groups, but noticed a problem which I have observed on a previous occasion shooting this rifle i.e. a significant shift in POI. Here's a link to last time I noticed it: [www.network54.com]
This time, I fired one shot which hit just to the right of the bull, then shot #2 landed 6" high. I thought I must have aimed at the next bull up by mistake, but shot #3 landed next to #2, despite the POA being correct. This is what the target looked like, with shot #1 at the bottom of the ruler:

I have ruled out the slugs as being a problem (they are Barnes Raptors, carefully crafted by Jerry's lads and very uniform in weight and size), and the same problem arose with an entirely different slug (EPP-UG's) last time. I chronied the 3 shots, and can rule out anything odd there, as the velocities were quite consistent: 627 624 616 (fps).
So I'm picking an internal problem with the elevation adjustment of the scope shifting in response to the recoil of the rifle....? The scope is a Bushy 4200 6.5 - 24x40 which I have had from new, and is securely mounted.
Any thoughts appreciated.
Cheers
Neil

Neil,

Looks like we have a science project here.

Since the slugs are consistent and you fired them over a chrony which proves they were fired consistently, they should have followed consistent paths to nearly the same spot given no huge changes in wind (like a 30 mph down draft for the first shot). Since you suspected POA and verified it to be correct on the third shot we can rule out POA as well. So that means there is at least one variable not considered and maybe more.

Was there a long delay between shot 1 and 2/3? I'm thinking of a cold gun being brought out shot and then lying in the sun for 30 minutes before the next shot. A scope getting hot could shift POI.

The scope may indeed be damaged, it happens.

If the scope is working then that leaves the barrel not pointing in the same direction every shot. Have you checked that the barrel band is snug and not cracked anywhere? How is the fore stock attached? Anything loose?

Any changes in grip between first and subsequent shots? I doubt technique but figured I'd ask as changing from a bi-pod to bags could make a difference.

Hi Neil,

If my memory serves me, the 909 is the same sort of frame as the early Shin Sung guns. The bottom air tube is shorter than the upper one, and is actually welded to the top tube at the rear of the bottom tube. There's a shim of steel between top and bottom tube for spacing, and that shim is welded to both tubes. This leaves the only tube to enter the receiver, as the top tube. Those tubes are also quite thin. Obviously, they are thick enough, and I'm not making any accusations here, but the fact that they are thin allows them to move depending upon how much air pressure is in them. If you do the math, you'll see that the smallest most within the gun could easily translate to your 6" movement at 50 yards.

Probably ten years ago or more, the guys were using these guns as FT guns. They were extremely accurate in .177". It was noted, however; that point of impact shifted as the pressure inside the tubes changed (although the velocity remained unchanged). Some guys were altering the front housing which connects the two pressure tubes and the barrel. The alteration was to "free" the barrel from it's union with the pressure tubes - thus the pressure tubes could dance under the barrel, without taking the barrel with it.

Now, they got away with this because the .177" guns were shooting 20 fpe ... and recoil was nearly non-existant. The 45 will not be the same. I suspect you'll have to experiment with the point of impact, as it relates to the reservoir pressure for that shot. Chart it. Confirm that it's consistent, then compensate for it. Either start your group at a different psi, or compensate for point of aim per individual shot as it relates to remaining psi.

Gary

Jerry and Gary, some very interesting thoughts, thank you. I think I can safely rule out wind, as it was about the same for all shots. Time between shots was quite short, and I rested the same way for each shot. But maybe I was a bit quick to condemn the scope, as this particular scope, when mounted on a Revo I once owned, helped me shoot the only sub MOA 100 yard group that I am aware of from a Revo

So, unless it doesn't like the recoil of the 909, maybe the scope is fine....
I'll investigate some more around whether there are any loose bits, and will explore the psi/air tube factor.
Much obliged
Neil

Neil,

I would think the quickest way to test the tube warping theory is to fill to the same PSI you used for the first shot in the original post, fire a shot and then refill to the same pressure. If the next shot falls close to the first then you probably don't have a scope problem. I'd refill again and take a third shot and if it also fell along the same elevation as the first two from the same pressure, then I would shoot again and see if the POI shifted upward. Ultimately, if the tubes are bending some at the initial fill pressure, you might want to start at a lower pressure and give up a shot on every fill for having the remaining shots consistent. It appeared to me that your 2nd and 3rd shots off a fill were at the same elevation so starting at a lower fill pressure might be the easiest solution.



Edited 1 time(s). Last edit at 02/10/2009 06:30PM by Jerry.

Thanks Jerry, I'll try that.
Regards
Neil