177 M-rod POI shift may be gone....

[RayK]

I've been experiencing a POI shift in my Marauder rifles - most pronounced in my 177 and my refurbished 25. I have long suspected that barrel harmonics, the stock, and cocking the gun have contributed to this shifting POI. I tried quite a few things to eliminate it (even gluing most of my gun together with blue loctite), but nothing really worked until maybe now.

Once you got a good barrel and pellet it likes and are not getting "fliers", the POI shifting will look like this. You shoot 3 pellets into a single hole, then the 4th, 5th, 6th, pellet go into a single hole away from the first hole, then it will shift back to the first hole for pellets 7, 8, 9, and 10.

The last round, I got a spare stock from Dick Strever, sanded the track to free float the air tube (not the barrel), then bedded the action using JB Weld WaterWeld. It seems very solid in the stock and changing the stock bolt tension does not seem to make any change in POI as it did before. I noticed the POI shifting was reduced, but still happened.

Next I added masking tape shims to the sides of the trigger group to make it more secure in the stock and generously lubed the o-ring around the muzzle shroud spacer.

Immediately, the POI shifting disappeared and I have not seen it since. I first noticed this about a month ago, but I was waiting to post until I could test it more times in different conditions. So far, it seems good.

I am not sure which of these modifications is necessary, so in my next rifle (my refurb 25), I just did the o-ring lube and trigger group bedding. That one is too loud to test in my indoor range, so I have not yet been able to confirm if it is shifting or not. When I do get outside, there is always wind, but it looks pretty good so far.

I just shot a 10 pellet group into a single hole at 10 yards. Yes, 10 yards is not far, but the POI shifting shows up in 10 yards when it is occuring. An accurate pellet gun shot well from a benchrest at 10 yards with a good pellet and matching barrel should be able to shoot an entire magazine into a single hole about the size of one pellet at 10 yards. I've now done this about 10 times with my 177 Marauder.

Other notes, the barrel is free floated as Crosman designed it - nothing in the barrel band except air. The shroud is snugged down on the rear shroud mount, but the mount is forward so the shroud does not put pressure against the breech.

That barrel only likes AA 10.34 grain pellets. All others I've tried have fliers and groups open up. This is a stock Crosman barrel (my 4th in this gun trying for accuracy) and it was recrowned and inleade and transfer port polished by an experienced airgunsmith.

Ray

[Motorhead]

Lol ... Vasolene on those things that slip & slide is an OLD SCHOOL TRICK dating back to the 1950's, who ah thunk

Good find Ray .. maybe having the sticksion of the stripper o-ring within shroud being loosy goosey kills some tension between them and deadens harmonics ???

Personally have always greased the stripper o-ring making shroud on-off chores easier ... dumb friggen luck if it actually helped accuracy too

[RayK]

I do suspect the lube is allowing the barrel to move back to the same spot after the barrel does its harmonic movement.

Ray

[jhm757]

RayK

Read your post and had to give lubing the air stripper o-ring a try.

It was just before dark so there was very little wind, I regularly shoot the online 30 yard rested air rifle match, and usually try to shoot in the evening when the wind calms down.

So I go out and pull the shroud and put a generous amount of superlube on the air stripper o-ring.

Now my .25 SynRod is normally a pretty good shooter, my best score until today was a 195/200 w/5X's and I consistently shoot in the low 190's.

But after lubing that o-ring I was able to shoot a 198/200 w/3X's. Best score ever!!!

So I think you may be on to something Ray. My rifle shoots pretty good anyway but every little improvement helps.

Thanks for all you do to figure out how to make these rifles better than they already are.

Jim - jhm757

[Doc]

I would place my money on the bedding. Try it again with the o-ring dry and let us know.

[steveinla]

That actually makes sense about allowing the barrel and shroud to move freely. It never ocured to me that the shroud might be sticking to the barrel. Good for you Ray!

[jhm757]

I would place my money on the bedding. Try it again with the o-ring dry and let us know.

I have shot a lot of targets trying to improve on my prior best of 195/200, and have shot a lot of 195's but never been able to do better.

My .25 SynRod is all stock free floated barrel. No bedding. Stock bolt just snugged up not overtightened. It has always been a good shooter. But just a little superlube on the air stripper o-ring and I shoot my best score ever improving by 3 points, that is significant to me. I have got to believe the lube helped tighten up my groups.

I will have to shoot some more targets to see that the improvement continues and the 198 was not just a fluke, but it would seem a strange coincidence that I shot my best ever and it wasn't the lube helping.

Jim - jhm757

[Doc]

Now put that barrel under tension and I bet it will do even better.

[RayK]

I bet it will.

I think all the things I did helped, but the last two things done at the same time seemed to make the biggest difference. The barrel vibrates after each shot and the dry o-ring might tend to hold the barrel within the shroud. I could tell something was moving the tip of the barrel. In an unmodified stock, I can see the entire airtube flex as I tighten down the stock bolt. I had gone to very loose stock bolt in un-bedded guns.

I'm a convert to bedding and I think free floating the air tube is worthwhile too. I plan to do it to all my Marauder rifles eventually, but it is a lot of work. I love the feel of the solid connection between stock and action.

Ray

[jhm757]

Now put that barrel under tension and I bet it will do even better.

What exactly do you mean to put the barrel under tension?

Are you talking about using something between the barrel band and shroud or set screws thru barrel band to hold shroud?

Jim - jhm757