How long is safe for the rifle length? 20-22"???
To change the system I would just need to get a different gas tube? Assuming the gas block was the correct diameter I could use it correct?
18" is as short as I would personally go with a rifle-length gas system and even then, depending on what ammo you'll be running and other aspects of your particular setup, that can get hairy without an adjustable gas block, but generally will work fine. 20" is prettymuch ideal for a rifle-length system.
As I said in my earlier post...YES...you WOULD need a different gas tube to match the gas system length that you decide on (mid-length or carbine would be advisable for a 16-17" barrel...if 17", then you might consider an intermediate length as well which is in between a rifle and a mid-length gas system). That was the "$10" figure I gave in my estimate on the conversion of your existing setup for "new gas tube in the proper length."
ASSUMING that the barrel is the same diameter where the new gas port would be located as your existing journal, then yes...you could skip the contouring and use the same block without issue. If it is not, then you are back to having the barrel contoured from the journal to the muzzle. Depending on the barrel size...you can likely elect to have it contoured to mate up with your existing gas block, thus saving a few more dollars on the switch to a shorter barrel so you don't have to buy a new block.
Again...it CAN be done, but for all the headache and time involved, I'd skip it and just buy a new barrel that suits your wants/needs in a shorter version. 1:7 twist 16" barrels, depending on what mfg, contour, gas system length, etc. you want, are readily available for non-gouge prices even in the current market (non-gouge being <$275 depending on the above...more for SS, specialty contours, match barrels with matching bolts, etc.).
