Horizontal poi shif...
 

Horizontal poi shift?  

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mata777
(@mata777)
Joined: 1 year ago
Posts: 6
2019-06-17 19:41:17  

I’m having some issues with horizontal poi shift with 2 different guns at 10m (my usual rat range), a crosman 1322 with 14.5” barrel and a 2250 with 12” barrel, one hole groups wander off 1/4” to the right using two different scopes a day or two after sighting in (bug buster 6x32 and 3-12x32) both scopes are new.

I’ve tried two different leapers mounts with no success. I’m not new to shooting and am aware of keeping my head position consistent when sighting in and checking for loose screws. Have also tried 3 different types of pellets (rws, jsb, and crosman)

I would appreciate any help, thank you. 


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Hector J Medina G
(@hector-j-medina-g)
Member of Trade
Joined: 2 years ago
Posts: 286
2019-06-18 10:13:37  

Sounds to me that you are canting the guns. If you are right handed, then canting them to the right will give you a POI shift to the right, at 10 meters the vertical component wouldn't be too noticeable.

Just an idea.

 

 

 

 

 

HM


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Charlie Sierra
(@ranger2bn)
Joined: 5 months ago
Posts: 79
2019-06-18 10:28:20  

User error. Put a bubble level on there, you’ll see it’s you. 

Sled mount the guns or if you done have one vice mount them. It’ll tell you real quick if it’s you or the gun. 

Pro tip: it’s you. 


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mata777
(@mata777)
Joined: 1 year ago
Posts: 6
2019-06-18 10:58:16  

Thanks everyone. At this point I’m blaming the crosman stock design and myself. Will be buying a bubble level. 


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James Perotti
(@jpsaxnc)
Joined: 2 years ago
Posts: 74
2019-06-21 07:23:12  

The .22 cal. Crosman barrels, are  sensitive to temperature changes. I have gotten a 2.5" shift in POI @ 50 yards, with a 20 degree change in temp. with 3 different Crosman barrels . Just a thought. PS. the barrels were 21.5" long. HTH


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nervoustrigger
(@nervoustrigger)
Joined: 2 years ago
Posts: 184
2019-06-21 10:01:21  

Regarding temperature sensitivity, an airgun itself or the scope (or the combination of the two) has been known to exhibit POI shifts perhaps as large as 2.5in at 50 yards, however I have never heard of a barrel being responsible for anything approaching that magnitude. For a 20 degree change, the extent to which the barrel’s dimensions or its physical properties change is infinitesimal.

Whereas something that alters the angle of the barrel (relative to the scope) even a tiny fraction of a degree will produce a significant change in the POI at a distance. For example, a stock (wood or plastic) may have internal stresses that causes it to distort ever so slightly when it warms up or cools off. That can push the muzzle at a slight angle and cause a big change in the POI at 50 yards.


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James Perotti
(@jpsaxnc)
Joined: 2 years ago
Posts: 74
2019-06-21 15:09:33  

Hi NT, I had 3 identical barrels, each had been straightened, same size transfer port, same length, same crown. The gun had been in the house at 70 degrees F. I took it outside and shot a couple of groups. Shot another group, and the POI had shifted to the left, and the group size had opened up. I went back inside and swapped out the barrel, went back outside and fired another group, POI was back to it's original place and the group size was back to normal. waited a bit until the barrel had heated up again, and the POI had shifted and group size opened up. swapped out the barrel again for the third cool one, and got the same results as the first two. I assumed it was the change in temp.


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Steve in NC
(@steve-in-nc)
Dealer
Joined: 2 years ago
Posts: 329
2019-06-21 21:29:43  
Posted by: James Perotti

Hi NT, I had 3 identical barrels, each had been straightened, same size transfer port, same length, same crown. The gun had been in the house at 70 degrees F. I took it outside and shot a couple of groups. Shot another group, and the POI had shifted to the left, and the group size had opened up. I went back inside and swapped out the barrel, went back outside and fired another group, POI was back to it's original place and the group size was back to normal. waited a bit until the barrel had heated up again, and the POI had shifted and group size opened up. swapped out the barrel again for the third cool one, and got the same results as the first two. I assumed it was the change in temp.

Hi, James. 

A question -- you mentioned the inside temp (70F), but how hot was it outside?  Lead expands more than twice as fast in the heat (16ppm/F) than steel (7ppm), so a rise in temperature makes pellets fit tighter in the bore.  It's certainly not a big effect -- only 0.000002"/F for .22!

Could such a tiny change in fit (e.g., 0.00004" = 40 micro-inches for 70F to 90F) account for your observations?


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James Perotti
(@jpsaxnc)
Joined: 2 years ago
Posts: 74
2019-06-23 10:59:16  

Hi Steve the outside temp. was in the mid to high 80's. I had only ever seen this problem with the Crosman barrels that came on the 2260. That's why I chimed in when the OP, said .22cal. Crosman barrels. I still believe that it was the temp. change that was effecting mine.


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Steve in NC
(@steve-in-nc)
Dealer
Joined: 2 years ago
Posts: 329
2019-06-24 07:32:59  
Posted by: James Perotti

Hi Steve the outside temp. was in the mid to high 80's. I had only ever seen this problem with the Crosman barrels that came on the 2260. That's why I chimed in when the OP, said .22cal. Crosman barrels. I still believe that it was the temp. change that was effecting mine.

I guess one test of the temperature theory would have been to take the gun back inside to cool off -- then see if the POI shift went away.  If the cause was temperature, it should have be reversible.  Right?


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RockDoc65
(@rockdoc65)
Joined: 2 years ago
Posts: 59
2019-06-24 11:05:58  

I don't mean to throw Crosman under the bus since they are everything they are meant to be. I believe the problem is; we expect too much of the design. A Crosman is basically a box of molded, stamped, drawn and folded parts held together with a couple dozen tiny screws. Some models even use a two piece receiver design and "sandwich" a bunch of parts in between hoping they will all stay aligned under screw tension and pumping pressure. I'm not faulting their design, I'm just saying they were never intended to be "bench-rest match" guns.

The problem only arises when we start throwing dozens of dollars worth of parts onto them and expecting performance to increase linearly with dollars. Face it, we've all owned at least one $350 Crosman in our lives or we wouldn't be here.

I would start by looking at any remaining plastic components or weak fasteners.


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