07-09-2017, 02:34 PM
My tale of woe and frustration with the NP XL 1100 continues. I examined the barrel that came with the gun and saw that the bore was much more uniform than the replacement. I pushed pellets through it and found that the breech was tight, there were a couple of somewhat less tight spots further in, that the point just before the muzzle was basically loose, and that the pellet would hang up right at the crown. The head of the pellet would literally hang up. When the head was pushed through the pellet would move freely indicating that the skirt was loose in the barrel until the skirt got to the crown whereupon it would have to be pushed through. I set about fixing this with a round headed brass screw and some valve grinding compound. After hours of that I decided to file the deeply recessed muzzle back past the point of the existing crown to try to get past the problem spot. Before doing that I decided to try lapping the barrel to open up the breech and get rid of the sticky spots. It seems counter-intuitive, reaming out the breech because the pellet was loose near the muzzle but I figured that the pellet was getting necked down by the tight spots so eliminating them would result in a better fit at the looser spots. The crown I would totally re-do after the lapping process.
I chucked a #8x32 tpi screw into the end of my cleaning rod, cutting off the head and filing flats on the exposed threads to give more room for the lead. I wrapped teflon tape around the joint where the screw joined the cleaning rod so that it sealed tightly against the bore. This left about 1" of the threaded portion of the screw sticking out past the teflon seal. I inserted the rod from the muzzle until the screw portion was about 1/4" below the breech opening. The trick is to heat the barrel, in this case the breech block, not too much but some, so that the lead would not leave too many voids, then pour molten lead down into the breech so it forms an elongated plug, the "lap", inside the barrel and formed around the screw in the end of the rod. This lap accurately takes the shape of the bore and rifling in the section where it was cast which darn well oughta be good for the rest of the bore too unless you've got real problems. Fortunately the lead contracts as it cools and is easily pushed out the end of the barrel a bit, just enough so you can cut off the mushroom end and cut some grooves around the circumference. Don't push it all the way out or if you do be sure to get it back in lined up with the correct grooves. Then put some lapping compound on it. Pull it back into the barrel, which takes some force, then work it back and forth, smoothing out the rough spots inside the bore. By spending more time at the breech than near the muzzle you can actually taper the bore from breech to muzzle. I of course still had that loose spot in the last inch or two before the crown. Nothing I could do about that but it was smaller now in the sense that the pellets remained tight over more of the bore, not getting swaged as much in the earlier sections. The crown itself was a real pain. Try as I might the pellets still hung up like the crown had a ring of teeth. I finally put compound on the point of a metal pen and worked it around to knock off whatever edge there was.
Finally after hours of work I had the gun reassembled with the original, lapped and re-crowned barrel and...drum roll please...I had lost another 30 fps. It did seem to group better until it broke the latest new scope I had put on, naively thinking I had fixed that problem. I'm about ready to throw this thing in the garbage. It is supposed to shoot lead pellets, I naively assume again that they mean the 14.3 grain Crosman pellets, at over 900 fps and I am now barely getting 800 fps with those pellets. If the spring is still putting out the original power then the power not leaving with the pellets must be left behind in the gun causing mischief like banging the scope around. If the spring is getting weaker then that leaves the scope issue with no likely solution.
So what I have done to the barrel has twice now reduced pellet speed across the board. The reason I bought the second barrel was that I foolishly read on the internet that it was a good idea to chamfer the breech where you put the pellet in. In fact a nice radius was the real deal according to some guy named Cardew. Well I did that and immediately lost 30 fps. I also separately tried seating the pellets just inside the breech. You really have to push on the pellet with a small stick and then the pellet lets go and winds up 1/8" or so further in than you want because of course it suddenly gets easy once the pellet skirt scrapes through. Well you lose a lot more than 30 fps when you do that so there must be a lesson there and I think it is that you WANT the pellet to stick in the breech until the pressure really builds up. If the pellet is free to slide easily then the pellet just moves away leaving nothing for the piston to push against sort of like failing to shift into high gear when driving a manual shift car. If on the other hand the pellet sticks for a while then the pressure really builds up, the piston slows to a stop with all its kinetic energy now stored in the high pressure air, and the pellet has what it needs to accelerate much faster than the piston if/when it ever does let go. So making it easy for the pellet to slide that first inch or so down the bore is actually a bad thing. Or so I have gleaned from the net. Except for that little trick about polishing the breech end of the bore. Dunno how that could work for anybody given how it did the opposite for me. Gee, thanks Mr Cardew.
So now I am contemplating how to put sharp edges back into the nicely radiused breech port. Give it some toothiness to hold the pellet a bit longer. Or swage the bore closed a tiny bit at the breech although that would make the pellet too small from there on unless the pressure is great enough to flare the skirt of the pellet out as I have read it is. Or get another new barrel. No word from Crosman yet.
I chucked a #8x32 tpi screw into the end of my cleaning rod, cutting off the head and filing flats on the exposed threads to give more room for the lead. I wrapped teflon tape around the joint where the screw joined the cleaning rod so that it sealed tightly against the bore. This left about 1" of the threaded portion of the screw sticking out past the teflon seal. I inserted the rod from the muzzle until the screw portion was about 1/4" below the breech opening. The trick is to heat the barrel, in this case the breech block, not too much but some, so that the lead would not leave too many voids, then pour molten lead down into the breech so it forms an elongated plug, the "lap", inside the barrel and formed around the screw in the end of the rod. This lap accurately takes the shape of the bore and rifling in the section where it was cast which darn well oughta be good for the rest of the bore too unless you've got real problems. Fortunately the lead contracts as it cools and is easily pushed out the end of the barrel a bit, just enough so you can cut off the mushroom end and cut some grooves around the circumference. Don't push it all the way out or if you do be sure to get it back in lined up with the correct grooves. Then put some lapping compound on it. Pull it back into the barrel, which takes some force, then work it back and forth, smoothing out the rough spots inside the bore. By spending more time at the breech than near the muzzle you can actually taper the bore from breech to muzzle. I of course still had that loose spot in the last inch or two before the crown. Nothing I could do about that but it was smaller now in the sense that the pellets remained tight over more of the bore, not getting swaged as much in the earlier sections. The crown itself was a real pain. Try as I might the pellets still hung up like the crown had a ring of teeth. I finally put compound on the point of a metal pen and worked it around to knock off whatever edge there was.
Finally after hours of work I had the gun reassembled with the original, lapped and re-crowned barrel and...drum roll please...I had lost another 30 fps. It did seem to group better until it broke the latest new scope I had put on, naively thinking I had fixed that problem. I'm about ready to throw this thing in the garbage. It is supposed to shoot lead pellets, I naively assume again that they mean the 14.3 grain Crosman pellets, at over 900 fps and I am now barely getting 800 fps with those pellets. If the spring is still putting out the original power then the power not leaving with the pellets must be left behind in the gun causing mischief like banging the scope around. If the spring is getting weaker then that leaves the scope issue with no likely solution.
So what I have done to the barrel has twice now reduced pellet speed across the board. The reason I bought the second barrel was that I foolishly read on the internet that it was a good idea to chamfer the breech where you put the pellet in. In fact a nice radius was the real deal according to some guy named Cardew. Well I did that and immediately lost 30 fps. I also separately tried seating the pellets just inside the breech. You really have to push on the pellet with a small stick and then the pellet lets go and winds up 1/8" or so further in than you want because of course it suddenly gets easy once the pellet skirt scrapes through. Well you lose a lot more than 30 fps when you do that so there must be a lesson there and I think it is that you WANT the pellet to stick in the breech until the pressure really builds up. If the pellet is free to slide easily then the pellet just moves away leaving nothing for the piston to push against sort of like failing to shift into high gear when driving a manual shift car. If on the other hand the pellet sticks for a while then the pressure really builds up, the piston slows to a stop with all its kinetic energy now stored in the high pressure air, and the pellet has what it needs to accelerate much faster than the piston if/when it ever does let go. So making it easy for the pellet to slide that first inch or so down the bore is actually a bad thing. Or so I have gleaned from the net. Except for that little trick about polishing the breech end of the bore. Dunno how that could work for anybody given how it did the opposite for me. Gee, thanks Mr Cardew.
So now I am contemplating how to put sharp edges back into the nicely radiused breech port. Give it some toothiness to hold the pellet a bit longer. Or swage the bore closed a tiny bit at the breech although that would make the pellet too small from there on unless the pressure is great enough to flare the skirt of the pellet out as I have read it is. Or get another new barrel. No word from Crosman yet.