M249 loss of fps

[Eeyore]

I have an A&K M249 that started its life as a MK II. I have since converted it into a Para. As it now has a much smaller barrel was I required to change the cylinder or piston head? I recall it shooting about 380 fps before not it shoots low 300's.

[lurkingknight]

what was the starting barrel length and the new barrel length?

>450 is a full cylinder
350mm-450 you can do cuts at the end
<350 you can do a 3/4

Though going shorter in barrel length to my knowledge is less of a need to change cylinders but you might see joule creep... hard to understand that one and I'm not sure if I understand the phenomenon myself... but the effect is essentially more joules with heavier ammo than expected at X fps...It's an effect you normally see in gbbrs using heavy ammo.

But if all you did was change barrels and hopup rubber and lost some fps, I'd wiggle the inner barrel and hopup rubber around and see if it won't seat better. .. it's more likely you introduced a bit of of a leak in changing the barrel.

[ThunderCactus]

changed from a 509 to a 363
so a loss is expected, just be sure the hop was installed and re-seated correctly
But you should have a ported cylinder

[Eeyore]

Originally Posted by ThunderCactus

changed from a 509 to a 363
so a loss is expected, just be sure the hop was installed and re-seated correctly
But you should have a ported cylinder

I do not have a ported cylinder in the gun currently. Can someone tell me exactly what a ported cylinder does? When it comes to aeg's I still have no clue which is why I like WE's.

[ThunderCactus]

Well I can't find that thread either
Anyway there's a set volume in your barrel that's filled with air from the cyl (duh)
Now if you got a full cyl and short barrel, the bb leaves the barrel when the piston is only (for example) half way forward, meaning it didn't reach max speed (therefore max pressure)
Whereas short barrel + ported cyl, piston gets up to its max speed very fast since it has no forward air resistance, then as soon as it passes over the port it delivers instant higher pressure air to get that bb out.
That instant higher pressure air is also what inherently gives GBBRs better accuracy than AEGs since the bb leaves the hop rubber at higher speed (imparting more potential hop which allows you to use heavier bbs)

[lurkingknight]

there's a magical ratio of cylinder air volume that should correspond to the barrel volume... using that fancy fangled math thing... the ratio is somewhere around 1.8:1 cylinder:barrel if I'm remembering it right.

undervoluming obviously makes your pew pew more like bleh bleh... (tiny cylinder with longass barrel) my g36 suffered this issue when I sorboed it... it had the original 3/4 port in the cylinder, and the person I bought it from had a 363mm barrel in it... After sorbo install it was shooting 190fps with a 410 fps spring.

Not changing anything but the cylinder to a 7/8 (ported at the end) the gun started shooting 390-405 with the sorbo and nothing else done to it.

Now... moving beyond that sweetspot... you get something called joule creep.

When the ratio is matched (or so I've read) when you chrono your gun with .2s you should get as an example... say 1.48J I believe thats 400fps. The important number to pay attention to is impact energy in J... With a 0 port cylinder and a tiny 247mm barrel, you have way more air to be pushed than the barrel can hold like TC said. Sure, you can get this setup to shoot 400 fps with .2s.

BUT.

When you put heavier ammo in... say a .3 if your cylinder is matched properly to barrel, you would see a graph that's a straight diagonal line... you'd be at like 327 fps. (ammo weight inbetween the two would fill in that line... Joules stay the same while FPS drops. This is the ideal situation.

That's not necessarily what happens. You can in fact see HIGHER (>327)fps with heavier ammo in this setup because the BB is heavier and has more time to accelerate faster leaving the barrel. You know how gas guns have this issue in the morning and afternoon when it gets hotter? It's a sort of similar thing... I think.

Your higher fps in this situation would indicate HIGHER impact joules. You could theoretically have a projectile impacting with more energy than a .2 @400fps in an overvolumed setup.

I have not tested how bad you can overvolume and what sort of fps you can end up with doing it. Less than honest players will do this and use heavier ammo, their heavy ammo loses less velocity vs someone using the same heavy ammo with a properly ratioed setup... while their gun still chronos 400fps with .2s

It's sort of a way to cheat the chrono. What should be done is you should chrono the gun with a chrono that calculates the joules on the spot while using the ammo you're going to shoot... or do the math yourself with the weight of ammo vs fps recorded.

It's also a bit difficult to enforce a hard J limit with the ammo one is going to shoot though... Unless people cut their own ports (some of them do) You will get varying ratios as there are fewer cylinder types than barrel lengths, so you will have ratios that are higher or lower vs the sweetspot so you'll see slightly slower rounds or slightly hotter rounds. I can't imagine the variance in J to be that great... I think there was a procedural test of X cylinder with 1-10 barrel lengths and I suck at remembering numbers.