BNM Breech and shroud

[DANthirty]

Hi All i have fittted the BNM breech and shroud which is a treat to use (so smooth) and the build quality is superb, my dillema is that since installing the new breech and shroud my grouping is all over the place it  was shooting 10mm groups at 20 yards (all i can shoot the length of my garden!) but now seems to be all over the place  do you think this could be the shroud doing this? or the differant air pressure now due to the differant size breech, thanks Dan

[Crosshairs]

Hi All i have fittted the BNM breech and shroud which is a treat to use (so smooth) and the build quality is superb, my dillema is that since installing the new breech and shroud my grouping is all over the place it  was shooting 10mm groups at 20 yards (all i can shoot the length of my garden!) but now seems to be all over the place  do you think this could be the shroud doing this? or the differant air pressure now due to the differant size breech, thanks Dan

It could be the baffles take off the shroud and see if there are any marks from a pellet if so you out of alignment, also make sure your barrel is secured and every thing else should be checked to make all is secure and aliened. I'm no expert but this is were i would start.
                               Mike

[ped]

i'd say it was the shroud, try it without it if your group tightens up then it confirms it
also check that the breech isn't rocking when i fitted mine i found that the extended tp was a fraction long and the breech didn't tighten down properly
ped

[quickster47 †]

I would definitely try it without the shroud.  To me it sounds like a baffle or baffles are clipping the pellets.

Carl

[breakfastchef]

How far forward did you mount the front collar that affixes to the barrel? If it is not positioned properly, the baffles will be loose. I do not know if this could cause a problem, but just thinking of other things to check on top of what has already been mentioned.

Another thing to try is use a single shot tray or a pair of tweezers and load pellets manually. See if the groups tighten up. It is not uncommon to have accuracy issues using a magazine.

Lastly, if you have a chronograph, check pellet velocities over a string of 10-20 pellets. Look for speed variances.

[droid]

Hi Dan, we spoke earlier via eBay about the hammer pin.

Yes I would take the shroud off. It will just be a process of elimination.

Got to either the barrel or the shroud. 

[DANthirty]

I tried it without the shroud and groupings was a bit better but what I did notice when the shroud was off the point of impact of the pellets had shifted at 20 yards it was 3" higher and 2.5" to the left does that sound right moving that amount just by taking off the shroud? I thought the shots should be roughly the same with or without the shroud?

[millipede]

hey, got any pics?   

[ped]

try fitting the shroud and turning it 90 degrees see if the poi has also moved then try shooting at 30 yds to see if the poi is still on the same vertical plane
i had exactly the same issues but i was using a shroud that i made myself, my problem was the muzzle baffle was causing the problems
ped

[DANthirty]

try fitting the shroud and turning it 90 degrees see if the poi has also moved then try shooting at 30 yds to see if the poi is still on the same vertical plane
i had exactly the same issues but i was using a shroud that i made myself, my problem was the muzzle baffle was causing the problems
ped

How did you overcome your problem?

[breakfastchef]

I tried it without the shroud and groupings was a bit better but what I did notice when the shroud was off the point of impact of the pellets had shifted at 20 yards it was 3" higher and 2.5" to the left does that sound right moving that amount just by taking off the shroud? I thought the shots should be roughly the same with or without the shroud?

The BNM breech is very thin at the transfer port and flexes/pivots at that point. So yes, I can see your POI being off with the shroud removed. Make sure your set screws are snugged in the barrel band.

You may want to return your gun to the previous configuration to make sure it still shoots properly. If it does, you can focus to the breech and shroud.

[DANthirty]

I feel like throwing this rifle in the bin it's doing my head in!!! I've had it apart god knows how many times  I can't even get good groupings with the shroud off , I will have to put the original steel crosman breech on tommorow and see if I can get some good groupings again, just can't tell what's causing it. Wish there was someone local who could take it off my hands and sort it I don't mind a bit of tinkering but its starting to get the better of me!

[crossliner]

I feel like throwing this rifle in the bin it's doing my head in!!! I've had it apart god knows how many times  I can't even get good groupings with the shroud off , I will have to put the original steel crosman breech on tommorow and see if I can get some good groupings again, just can't tell what's causing it. Wish there was someone local who could take it off my hands and sort it I don't mind a bit of tinkering but its starting to get the better of me!

I would be mad myself if I were in your shoes Mr. D30..The BNM is not exactly cheap..Mr. Woody had the same issue w/ his BNM if I remember it right..I hope you sort it out 

[breakfastchef]

Did you try any of the things I listed in post #5?

I went through a week of troubleshooting a problem with loss of pellet velocity with a BNM breech/shroud. It took some filddling and thinking, but I got it sorted out. Matter of this nature must be dealt with by process of elimination.

[ped]

How did you overcome your problem?

i didn't buy the bnm shroud prefering to make my own and was trying an air stripper idea to divert air rearwards which seemed to be causing an uneven pressure wave that was affecting the pellet as it left the muzzle and was entering the forward ldc chamber
i also fitted a barrel band at the front of the breech so it pushed down on the shroud boss i also tapped the main tube m4  and used a m4 csk bolt instead of the 4-48 standard one
the tp gasket was changed for a slightly thinner one made from a piece of poly tube
ped