I just built a 635 SBR clone.  It uses a Colt 635 parts kit on an RRA receiver with removable mag block, modified Colt hammer (auto-trip spur ground off), DPMS trigger and safety.  It has been somewhat unreliable with Wolf, Blazer brass, Winchester white box, and American Eagle FMJ.  Completely unreliable with Blazer aluminum.  The Colt mag works much better than the C-Products mag but it still jams a couple times per mag with the Colt and about every 3rd round with the C-Prod.  The mag block is lowered all the way to the mag catch.  Failures are always failure to eject.  I bent the jector slightly inward and it is sliding neatly along the notch in the bolt.

I ran it about 50 rounds with a standard carbine buffer and it worked 100% with the Colt mag, still horribly unreliable with the c-prod.  I guess I'll pony up the extra cash for Colt mags for now.

Am I risking any harm to my SBR receiver by running a lighter buffer?  Any other things to try to get this running better?  Is it just weak ammo that makes it work with the light buffer but not 9mm buffer?  I'm using all the springs that came with the 635 demilled parts kit.  It has been completely dissassmbled and lubed.

Anyone?  Any ideas to get this running better with the 9mm buffer?

just incase you didnt know the newer(not sure when they started) colt mags are C prod mags the older colt mags were metalform mags

Breaking bolt catches and cracking/breaking the receiver extension tube (buffer tube).  Not from first had experience, but from reading is where this info was gleaned.

have read accounts of egging the holes in the reciever (hammer pin) and on FA setups breaking the hammer pins. Orgional Colt set up was hardened stainless steel pins (hammer and trigger)

I should have been clearer, the Colt mag is a Metalform.  My pins are Colt stainless steel.  I got an idea off another forum to check to see if the next round in the mag is knocking the empty off the extractor before it makes good contact with the ejector.  I hope to have time to test it some more this weekend.

Such a finicky beast, I had hoped to avoid most of this my using the Colt parts and RRA lower/mag block.

It's kind of tough to diagnose your problem without seeing the gun.  It could just be weak ammo and after a few hundred rounds, it might break in nicely and work just fine with a 9mm buffer.

On the other hand though, WWB is usually pretty decent ammo and not underpowered - it should cycle the gun just fine with a 9mm buffer, even a brand new gun during break in.  It wouldn't hurt to re-check the lower and also check the upper and make sure the ejector and feed ramp aren't dragging the bolt and the action cycles somewhat smoothly (nothing binding in the buffer tube - the action spring is sometimes a tight fit on the 9mm buffer so check that).

As far as risk to your receiver running a light buffer?  Who really knows?  Technically, every time you handle or shoot it, there is some element of risk that a part could break.  In reality though, how much sooner would/could it fail?  I don't know that anyone has tried both ways and can say conclusively that one way will fail X number of rounds sooner/later than the other.  I have not seen a buffer tube fail in person.  Come to think of it, I'm not sure I've even seen a pic online of that either.

As far as your theory on the round in the mag knocking the case off during extraction, I have seen that with some mags, but not with Metalform mags.  If your fired case is falling off prematurely during extraction, you may want to check your extractor tension and possibly clean the extractor tunnel in the bolt.

When you say you are using a de-milled parts kit, are you talking from a lightly used demo gun, or from a heavily used training gun?

It's a used PD gun.  Lots of handling marks and was quite dirty when I got it but the parts themselves look to be in good shape after a thorough cleaning.  I wouldn't expect anything needs to break in at this point.  The extractor was removed and it and the channel were cleaned very well.  There is some wear on the hammer face but other than that, the internal parts all look just lightly used.  Extractor tension seems fine when I put a casing on the bolt and give it a couple shakes.  Some cases eject about 5 feet straight right out of the gun, some just dribble out the port and land on the table, and some stay in the gun, causing the jams.  I'll check for clearance on the feed ramp as well.  The action seems to cycle very smoothly both by hand and during firing.  I can't feel the buffer spring binding when I charge it.

I don't really mind honest wear on a gun and sometimes things break, that's the price of not having a safe queen.  I just didn't know if there were going to be known issues by using a lighter buffer.  Since it's just a semi, I doubt it'll see enough hard use where wear would be a problem unless the light buffer is known to actually cause specific issues.   Which is another way for me to say I'm OK with some accellerated wear, just not outright abuse, UNLESS that will cause harm to the registered lower.

Thanks for all the replies so far.  It looks like I will have some time Saturday to take it back to the range for more testing.