I don't have the plenum yet (on order). I originally had the .25 with 600mm barrel and i shot alot of steel, spinners, dueling trees , plates ect. At 50 yards it will deform airgun steel targets (crosman squirrels) shooting jsb MKII 34grn at just under 900. So i upgraded all my steel targets to .22lr rated. Since then i have switched the gun over to 700mm .22 in anticipation of jsb knockouts, and the power left me wanting. My 22lr dueling tree won't lock up with a 26grn .22 at 900fps. I am still adjusting literally and figuratively. So if i can archive a tighter group in a commercially available (cheaper) slug vs the .25 ill be happy. Oh and as for the length that you want the 700mm runs quieter than shorter barrels so less sound moderation is needed, so the OAL of the gun might, within user preference, be the same.