Welcome to Irish sniping
I've talked with a couple of lads to reddit to update myself on what's best and it seems not much has changed. What I run is quite a budget build with a lot of DIY stuff but it can all be replaced with shelf parts.
First thing's first,you will pretty much have to play at AEG ranges so be stealthy or quick . Small 3x or 4x optic is good,no point in a massive 3-9x40 like I run. Adding inertia is a good idea,as long as the gun is balanced.
PDi-W hold is probably the better bucking for 1J VSR builds. I've been told the Maple leaf has the best sealing quality but is rubbish on it's own. The Laylax buckings have had spotty QC,needing material removal but are still good,though,generally people seem to like the 'split' buckings.
I run .28s and .30s,the latter being the max weight for the W hold in my experience.
If you want to keep the Stock chamber,you will have to do some work to the adjuster mech as it bends and causes uneven hop pressure. Very trial and error job. Consists of making thin metal spacers and sticking them around the adjuster and arm to increase rigidity. taken ages to get mine right and there's still a tiny bit of curve. Also you will need to invest in a dangerwerx Type B arm or the airsoftpro one to get the best out of the stock chamber.
Right now the Action army hop chamber is very popular. If you want absolute accuracy I recommend this. Pdi chamber is a bitch to adjust and the laylax one is expensive.
Good barrel always helps. PDi are the best option for Bolt actions. 6.05 bore Steel ones are good.
Barrel spacers of course help,you can make them out of masking tape,no different to bought ones. An extreme mod some american lad once did is pouring wax down his outer barrel to seal in the inner.
On the plus side,you can save some money since you can stick with the stock trigger mech. I'd replace the piston with one that has a steel catch like the AA or laylax one. SHS or PPS sear set is my choice. I'd chose the SHS piston sear as it's made of softer steel and is the cheapest and easiest wear part to replace.
Finally,goes without saying,standard sealing jobs are an absolute must. power output has to be as consistant as possible. The cylinder is an easy job. I am getting 100% full seal using PTFE tape,Stock Marui O-ring on a Laylax piston. Cylidner polished and lightly greased,head tapered and nozzle crowned.
The Action army chamber should have good seal. PTFE tape and maybe some gasket maker around the hop bucking and barrel joint is also a good option.