I couldn't find anything on Google in regards to the PDI W-Hold bucking in a Classic Army M15-type AEG specifically, but it seems to get good reviews in other guns. One person somewhere mentioned while his semi-auto accuracy increased, his full-auto accuracy decreased; this isn't a big deal to me as I only fire my airsoft gun in semi-auto.

IT's a direct replacement and in my opinion the best replacement rubber on the market

Thanks for that information Rusty Lugnuts, I just placed an order for one of those.

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So here's what I have ordered to increase the accuracy of my CA M15A4 Sportline Rifle:

1x Madbull Black Python 6.03mm TBB (509mm length)
1x PDI W-Hold hop-up rubber
4x BB Bastard 0.25g white BBs (2000 rounds per bag)

I spent quite a bit of money, but I'm sure it's worth it if it can improve the performance of my airsoft gun. I hope I made good choices in parts and BBs; BB Bastard BBs seem to get universal praise here and sound like the most accurate and highest-performing airsoft BB that money can buy (I'm trusting what you guys say).

30ft tells you nothing, you want accuracy at 160ft
Besides, you'll never get a true accuracy test without a steady rest and removing all human elements for inaccuracy

Like, I'm sure my VSR could hit a nickel at 140ft fairly consistently, but I sure as hell can't lol

I wish I could do accuracy testing and target shooting at larger distances than 33 feet (the approximate length of the longest area in my basement is 40 feet) while I'm at home, but I can't go out to my backyard as there's a child day care right next to my place (with kids playing outside during part of the day) and a neighbor whose top-floor windows kind of look over my backyard, so me going outside with my realistic-looking airsoft gun is just asking to have the police called.

Shooting 10m/33ft air gun targets is the best I can do for now to test the accuracy of my airsoft gun; I also use a stack of boxes where I can rest the barrel of the gun on and use that for support as I'm crouching and aiming.

well the key to your accuracy is what the BB does at the end of your range, so you'll never be able to even benchmark yourself properly in a basement. It's unfortunate, but that's the truth
But you should still upgrade the rubber. You can at least get your air seal tuned up for spring and test the range when you can finally get outdoors

I think it would be nice to have a slightly-upgraded gun when I actually do participate in the airsoft sport. Are there any known issues to using a full-length M16A4 airsoft gun in terms of size? (Almost everyone has an M4A1-based AEG)

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Also, once the stock CA Sportline internals wear out on me, would this drop-in gearbox and motor be appropriate to run on the 8.4V Ni-MH large type batteries (4000mAh Firefox and 3300mAh Elite) I currently own?

Gearbox: http://www.airsoftstore.ca/index.php...vddatkvagcot04
Motor: http://www.airsoftstore.ca/index.php...vddatkvagcot04

I know there are better more-powerful long motors than the one linked above, but I don't want to shell out more money on new LiPo or higher-voltage batteries that are needed to run those.

I also notice with the Lonex gearbox that the store offers a $5 option to add a sorbo+neopad to prevent premature piston failure, but which option (40oo or 70oo hardness) would be the best?

I bought a lonex gearbox and had the spring downgraded to a 110 before shipping. It shoots about 403 fps in a m4, in a m16 the fps would be higher due to longer barrel

Can the Lonex gearbox be run on a standard 8.4V Ni-MH large type battery though (with the appropriate motor)?

oh yeah, But your going to take 3 years to wear out the stock box. Who knows what the hot set up will be then

Lower-end gearboxes and internal parts, like the ones used by CA Sportline and other budget guns, can last that long with regular use?

I keep hearing that CA Sportline internals are crap.

The problem is not durability per say, it's consistency
I've known CA's to last anywhere from 1 game to 3 years without breaking.
And in G&P's (with good internals but very weak gear shafts), I've seen the gears fail after one game or last as long as 3 years of HEAVY use.

When it comes to the mechbox, just use it till it breaks

ThunderCactus is full on correct, Quality control is what separates the budget guns from the pricier ones. This doesn't mean that you can't get a good one cheap, it's just a bigger gamble. You will figure out pretty quick if you got a crap gun

Thanks ThunderCactus and Rusty Lugnuts on your insights to the inconsistency of mechbox quality and longevity from one airsoft gun manufacturer. I guess I won't be able to know how good of a Sportline mechbox I got until my first couple of real games. When it breaks, I've decided to just get a high-quality complete drop-in V2 mechbox to replace it, and use the Sportline mechbox to get hands-on practice with assembling, disassembling, and changing the parts on a V2-type mechbox. If you're wondering about why I'd mention drop-in mechboxes, it's because I don't have the skills yet to work with the internals of one, and so I'd be much more comfortable using a drop-in first for a replacement, even if I have to pay a bit more than a comparable mechbox built from scratch using separately-obtained parts.

About drop-in V2 mechboxes, is the Lonex V2 mechbox for the M16A2 (which comes with a non-ported cylinder for the long 509mm barrel) from Airsoft Store Canada the best quality and best-priced drop-in for my M15A4 Sportline gun, or are there any other good brands of reasonably-priced (ex. a Systema would be way out of my price range) high-quality drop-in mechboxes sold by other online Canadian airsoft retailers that I should also consider?

I replaced the stock barrel with the Madbull Black Python V2 509mm TBB, along with replacing the stock hop-up rubber with the PDI W-Hold rubber.

The first approx. 150 shots (with BB Bastard 0.25g) were very accurate out to 33 feet with nice tight groupings on the target (better than stock), however after that the shots became less and less accurate until they went well off target, despite me attempting to re-zero the sights multiple times to compensate. I pulled the new TBB and plastic hop-up unit out of my AEG and saw that the barrel had moved slightly, as the visible bulge from the W-Hold hop-up rubber (looking down into the barrel from the hop-up unit end) was no longer perfectly centered. I then discovered that the plastic c-clip that was supposed to secure the barrel wasn't on all the way, and for some unknown reason, I couldn't make it clip in all the way. Later, I even tried to refit the stock barrel back on, but the c-clip again didn't want to clip in all the way.

I was a bit disappointed, but hopefully the Lonex metal hop-up unit I ordered today from Airsoft Store Canada will be better than the plastic one-piece Sportline hop-up unit.