I am sorry if this has been already covered, but I am considering purchasing a Gen 4 Glock 21. Am I correct in that I can purchase a Glock 20 slide and barrel and that it will work with the Glock 21 frame ??? Also am I correct in that I should be able to purchase a .357Sig / .40S&W conversion barrels and use it in the Glock 20 slide. Please let me know if this is correct. Thank you for any info. God Bless Wanna kill these ads? We can help!