How big of a problem is leading in the barrel if I where to get one of those conversion kits that just replaces the bolt and carrier? I have a 20" Bushmaster (Chrome Lined Barrel and Chamber). Do I need to scrub the barrel after shooting 22LR or will any lead in the barrel get cleaned out with a few .223's through the barrel?

Thanks

DO NOT SHOOT IT BEFORE CLEANING.  Sorry for the shouting.  It will really lead your barrel and the chamber collects a large amount of fouling too.  These kits really make a rifle dirty.  But, on a positive note, the fouling/leading from the rimfire ammunition is pretty easy to clean out. It can be cleaned with just a few patches and a couple of passes with a bore brush.

5.56 ammunition is loaded pretty hot and shooting some with a fouled chamber or leaded barrel will send the pressures up.  Plus, any debris in the chamber is now baked and pressed on at about 60,000psi.

Here is a nightmare story for you.  At one time I did not have a dedicated .22 upper.  I would just put the kit into any of my .223 uppers.  I had about 7 different uppers with different barrel lengths, receivers, etc.  One day I went to the range, and there was a guy there who ran a police range.  He wanted to shoot full auto and offered to give me some pistol ammo if I would let him shoot.  I gave him the choice of a hundred rounds or so of .223 or unlimited rounds with .22.  So, he picked the .22 ammo.  He must have put about 750 rounds through the upper.  When he was through.  I removed the .22 kit and put the .223 bolt carrier back in.  Then, I either forgot to clean that particular upper or did not do a good job because I forgot I had let this guy shoot .22 through it.  At any rate, when I next went to shoot .223 through it, the pressure was so great that it blew the magazine out the bottom of the lower receiver and destroyed it, it forced the casing back so hard against the bolt face that the round and bolt were jammed and had to be removed by a gunsmith.  The bolt was destroyed. It also cracked the upper receiver.  The barrel was still good, and I used it on another upper.  However, I lost a Colt upper receiver, a Colt bolt, and a Colt magazine.  So, now I have a dedicated upper I built out of parts, so I don't have to worry about something like that happening.  Even if you clean your barrel after each use of .22 LR ammo, you can still get lead fouling in the gas port.  Then you have to knock out your front sight taper pins and remove the front sight to access your gas port.  If you use copper coated .22 ammo, this won't happen as easily but that ammo is not copper jacketed, so lead can still get deposited.  

Charles Tatum
Alamo Professional Arms

Yikes!

The instructions included with the ceiner "Atchisson" conversions prescribe cleaning the chamber, at least, after removing the .22 conversion and before firing .223 again.  At least one magazine of .223 between every 500 rounds of .22 to flush out the gas system.  As always, verify the bore is free of obstructions before firing (but especially before firing .223 after .22).

But I don't know that any of that would have helped in your situation, macv - that's the first instance of a kaboom firing .223 after .22 I've ever heard, and will probably cause me to double check and clean more thoroughly in the future when I've been using the .22 kit.