I believe you're right about not knowing which gun shot KR. I don't recall or have in my notes any information as to what caliber was used.
I base my conclusion of only three guns used for the murders based on this: At JW confession hearing , AC said JW gave them info on the weapons used and vehicles used. In the supplemental discovery 6/21/21, there are three guns and a knife listed and there are two vehicles listed. I am assuming that--since AC only listed three guns and two vehicles in that supplemental discovery document--those were the only guns and vehicles utilized. As for the guns, I back this up based on Agent Scheiderer's testimony at GW4 bond hearing 8/31/20. He stated three calibers of firearms--.22, .40 and .30 caliber.
Granted, there could have been a fourth gun that was the same caliber as one of the three specifically mentioned (.22, .40 or .30). But if that were the case, I think AC/Agent Scheiderer would have addressed that issue at the bond hearing and stated there were four weapons.
All JMO.
Link to confession hearing video. At 1:49:43 AC says some of the information provided by JW was new. "In addition to the information that Jake provided us, what he told us led us to discover some evidence that had yet to be recovered, specifically the weapons that were used in these offenses."
6/21/21 supplemental discovery:
Under DNA: knife from concrete 5/14/21
Under latent prints: knife from concrete /14/21
Under FWF: ATF Etrace 1911, Glock, SKS
Ford F-150 pictures
GMC Sierra pics