Several months back I purchased a complete .22 upper assembly from the equipment exchange to go on a spare complete lower that I had laying around. Once I got it in hand, I took off the faux gas block and carbine guards and replaced it with an MI SS tube. After I had done that, I took out, and it shot great. Accuracy was exceptional and there were no problems or issues to speak of.

Fast forward to a couple of weeks ago, and there was nothing resembling accuracy, and 80-90% of my rounds were keyholing like crazy. I did not change anything between these two range sessions. After having the keyhole issues, I looked down the barrel and it doesn't look right, though I never looked down it wrote, so I honestly don't know what I'm supposed to see with the rimfire system. Anyway, there is an obvious offset in the breech end of the barrel, and it is plainly visible regardless of which end you look down. Is this normal, or is my collar/ barrel suddenly misaligned?

I have a good deal of experience with AR's, but absolutely zero experience with the dedicated .22 set ups. Thanks in advance for any help/ advice.

Could it be a gnarly lead build up?

Yep! Just answered my own question. I tried to get a cleaning rod through it and it wouldn't go. So I lightly tapped the end of the rod and it broke free, knocking out quite a chunk of lead in the process. I'm gonna let it soak and then put her back together, I think we're good to go now. Thanks.

Shoot copper wash ammo. Winchester 555 and Federal 550. CCI Mini Mags also work well. Leaves less lead build up.
Dave N

Ditto to Copper Washed Bullets and it won't happen as long as you give a light barrel cleaning. Over cleaning isn't a good
thing and can damage the barrel much more than shooting thousands of rounds.
Dave S

Yes I learned my lesson with my drop in conversion. Never had any keyholing but every time I cleaned after shooting CCI Blazer I got big slivers of lead out of the barrel.

Thanks guys. I just bought a couple thousand mini mags, so I should be good to go from here on out.

Okay, clear it up for the rest of us, OP.
Is there really something wrong with your barrel or is the lead buildup in the 'shit happens' category?
BTW, can't remember the last time I put a patch thru' the 15-22....
Moon

ya put a patch through when it starts keyholin', any sooner is close to OCD.....    
  seriously, I'm with you, Moon, when I can't recall when I cleaned it last, it's time to clean it, AFTER the next shooting session.  got's ta have ya priorities right

please explain this to me.
it would be nice to justify not giving all of my guns a super through cleaning every time I take em out

This cleaning of .22 rifles draws out a lot of different opinions. Well, its a free country (for now) so I will express mine. I have been shooting .22 rifles since the late 1950's and since that time have put more rounds down the barrel on these than I could even count. I mean there are years when I though so many thousands I don't even want to count!

During these years of use I have learned the hard way that I should CLEAN and inspect the barrel of my .22 rifles and pistols.  How did I learn? By having my accuracy go straight to hell. By having firearms that had been working great to descend into terrible operations, etc. Yes, I know some of you will swear this cleaning thing is NOT needed. But on the other side of the coin are so many of us that have had major issues due to the build up of lead or other filth in the barrel or action. On one range day with a particular ammo I fired a few hundred rounds and actually had the bore of the firearm decrease to 50% of its normal size due to leading. This particular brand of ammo (which I actually threw away!) was so bad it leaded up four of my .22 fire arms over a short period of time. The one pistol I had to take an old cleaning rod and hammer it though the barrel to push out the lead. The chunk of lead that came out was twice the weight of one of the bullets that I was firing. When I saw it I realized why a normally great shooting firearm had been reduced to spiting out bullets all over the place. I learned my lessons well: Don't shoot xxxx ammo in your firearm. Don't want to offend any fans of this brand so it will remain nameless. Don't want a Thunderbolt to come flying down out of the sky and hit me!  <wink..wink>

So several times I have had good firearms go really bad over leading. You learn to avoid certain brands. You should also learn that every time you shoot more than a few hundred rounds of .22 it would be a good idea to take your light and look down the barrel. I just run a clean patch through it and look for the amount of dirt that comes out and then look down the barrel to insure it is clean. With good ammo that is the end of it. Again, even at the range I will do this every few hundred rounds.

Over cleaning is a different story. When I "clean" my barrel I use a soft patch and a little Hoppe's #9. A little on a patch then follow up with a clean patch. If it comes out dirty I repeat the process. This will NOT harm your barrel. Over cleaning is when people use stiff brushes, such as the stainless steel ones, and just take it up and down or round and round until it wears the steel of the barrel. For a normal firearm that is using good quality ammo a simple patch will clean up the barrel. Should you find a lead build up then move up to for the brass brush. Brass brushes are far better than stainless or regular steel ones. They don't wear your bore so much as do the SS brushes. If the lead is bad there are commercial products out there that work better than your normal firearms cleaners. Beware that these are some powerful mixtures you are working with. Don't take my word for it, just wait until you open some of these cleaners up. The fumes will burn out the inside of your nose. I have only had a couple of barrels get this bad.  

If you just use a basic patch and keep the firearms clean and carry out regular inspections on the internals, you should be good to go forever.

I have some 22"s that get the barrel cleaned maybe 10,000 rounds later and then I have a few that need to be cleaned
after 500 rounds or they will go to hell for groups because of lead build up.
The make/quality of the barrel and how polished the bore/rifling in the barrel makes a big difference along with ammo choice.
I have also seen chamber jobs that are so far off concentric to the bore that leading is more often too.