fill air tanks

I would suggest you look at a compressor. A decent one will completely fill your gun each time, where a dive tank will loose headpressure after each fill and then of course need a trip to the dive shop to get filled. Then there is the testing fo rthe tank that will happen every year that costs around $70 -$90. Some people think you don't need a test if it's not for breathing use, but it does need to be in test to be legally filled.

Get a compressor that works on 12 v and it means you can fill it in the field off your car battery. I have a GX CS4. Paid around $900 landed, but I see Hunting and Fishing might be selling them under Gamo label with their new PPCP 

(plastic precharged Pneumatic) 

I also have an earlier Airbow compressor as a backup, and haven't used my dive tank for a few years now.

The Gun City pump (GX CS2) is OK, but air cooled and heats up quick. Heat kills those wee pumps.

Be prepared to pump for maybe 10 minutes, and allow to cool. Pump for another 10 minutes. Can get tedious.

The next step up is the CS3 which has a fully enclosed case but the same guts as the CS2...so not much better at all.

Better of the lot is the CS4. It's water cooled (has a built in radiator). I sold mine recently for $1000 to give you an example. They are rated to 5½ hours continuous running.

I sold mine to get the Hill Evo-310, and pleased that I did. Only use it for gun filling, and it will outlast me I reckon.

For the GX CS2/3/4 etc you get a better price by buying direct from Alibaba (not Aliexpress). The Aliexpress sellers are sometimes selling a clone...and they have no shame.

For the money that GC want I'd be looking at a genuine Yong Heng (water cooled). Needs a bucket of water to pump from for cooling. Needs regular maintenance (yearly probably)...seals, grease, etc.

Yeah, sod the scuba tank filling guys. Damn dear per fill, plus the yearly test, and the 5 year test, etc.