Winkzy, you said it yourself.. the bbl length shouldn't have any effect - for the type of use you're intending on doing..
If there are any 'efficiency differences', they'd be so minor that I doubt that you'd even notice..
For hunting / pesting / walking-and-stalk styles of shooting, I'll just suggest going with the shortest configured option.
Especially if you're going to be adding another 6" or so by using a suppressor.. That said, I'd say 600mm barrel..
I thought about this really quickly also when it came time to pull the trigger on my Impact MK-II. Mine's in .22 cal.
Same deal, I went with the 600mm bbl length and figured I'd do the same if/when I get the larger cal conversions.
The guns offer so many shots per fill anyways, that adding more bbl length just seemed counter productive..
And if I had a better choice [personally] I would have opted for a "Compact", but, they weren't out yet, and I'm impatient.
Suppressors - I have a few DonnyFL Ronins and an Emperor V3, as well as the 6.26 Extender..
They've been in rotation on about 15 different PCP rifles, and they all work Amazingly.
Love em', and Highly recommend them! As for scopes, there's too many different options..
25+ years of shooting and it wasn't until' about 2 years ago that I found out that I liked first-focal-plane better than second..
I sold almost every scope I had in SFP, and grabbed a few FFP models made by Discovery: [
Discovery Optics VT-T
6-24X50]
They worked Great, were priced right, and I was hoping to get a bunch more.. annnd.. then they disappeared from the market.
I just threw a Bushnell
Nitro 6-24x50 on mine to try it out.. Great scope. Much clearer image-quality. I was tempted to get more..
Annnd.. Now those disappeared too. Everyone usually just recommends Hawk, Vortex, and probably now Element..

Sam -