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Guide: Lowering FPS for field limits

Posted: Thu Feb 05, 2009 12:51 am
by Moondog
How To Guide to lowering your AEG's FPS
by Moondog

FPS limits on fields are important not just for fairness but because of safety, especially on CQB fields where firefights between players can happen in 20ft or less. Many CQB fields have FPS limits of 300fps which wasn't a problem when most AEG's were built to Asian specs (300-350fps). But the trend in Chinese made AEG's has been to pre-upgrade them to American field specs with some stock AEG's shooting 350-400fps and reports of some stock weapons shooting above 400fps out of the box!

That's when you need to downgrade your AEG.


COMPRESSING YOUR SPRING
All new springs weaken with use. Over time you'll notice that your AEG's FPS drop. If it shot 400fps out of the box, by the end of summer you may chrono and discover that it now only shoot 380fps. This is known as "breaking in" a spring or your spring "settling down".

The same thing can happen if you accidentally keep your spring compressed (half-fired) overnight or in your closet for days. This is why need to fire your AEG twice in semi-fire before putting it away (see Beginner Airsoft). This makes sure your spring is fully released and reset. Keeping your spring compressed weakens it.

You can use this 'accident' to intentionally weaken your spring. You can half-fire your AEG by pulling the trigger just enough to compress the spring but not release it. You will hear your motor pull the spring but you need to release the trigger just before it can fire. It takes a little trial and error.

Keeping the spring compressed overnight can lower your FPS by 20-80 fps. If you're worried you'll weaken it too much, you should check it every hour or so with a chrono.

A word of warning: Keeping your spring compressed puts a LOT of stress on your gearbox. Remember, gearboxes are only designed to take the load of a compressed spring for short bursts, not continuously. So using the compressed spring technique is not without the danger of breaking a piston, geartooth or other internal part. This is especially so with very strong +400fps springs. Don't say I didn't warn you.


REPLACING YOUR SPRING
The best and safest way to lower your FPS is to replace your spring with a lower power spring. An M90 or a PDI-100% spring are 280fps-300fps springs. M100 or a PDI-130% spring are designed to to power your AEG to 300fps-330fps. Your results may vary.


CUTTING YOUR SPRING
Before upgrade springs were readily available, airsmiths commonly cut a coil off an AEG spring to weaken it. You'll need a strong hacksaw or a metal cutting wheel dremel tool because springs are tough. You may only need to snip off 1-2 coils to get your spring down 20-30fps.


SHORT STROKING YOUR PISTON
An advanced old-school mod to increase ROF was to grind off the last or last few teeth from a piston and/or your sector gear. By realeasing the piston early, you will also lower the FPS. Shaving off a tooth can reduce your FPS by 10-20fps per tooth. See guide here: http://www.popularairsoft.com/stories/p ... t-stroking


WIDE BORE / SHORTER BARREL
An overlooked method to lower FPS is putting in a shorter stock barrel on your AEG. Stock TM, CA and some LPEG barrels have an inner diameter of 6.08mm which can lower your FPS by 5-10fps. Buying a very short barrel also lowers your FPS (~5fps) because your BB spends less time under compression before exiting the barrel.


SPARE CQB GEARBOX
Some AEG design allow for easy access to your gearbox such as the AUG, G36, certain M16's and AK's. If you have the means, buy a spare gearbox that is setup for low FPS and simply swap it out for CQB fields.


AIRLEAKS
An unorthodox method to lower your FPS is intentionally introducing air leaks to your gearbox setup, either in the piston, nozzle or hop-up. Depending on the AEG model even a single millimeter shift in the position of your gearbox can drop your FPS. If you have an upgraded AEG, you could replace your o-ring airseal nozzle with a stock or loose fitting one.)



UNACCEPTABLE METHODS
The methods outlined below are not recommended or advocated but are known practices of reducing FPS in an AEG. In some cases they're outright cheating and a listed here to make you aware of these tricks if you actively chrono your games. The whole point of lowering your FPS is for safety. Validating your AEG for a game is secondary to that.


HEAVER BB's IS CHEATING
Using heavier BB's does lower your FPS but the energy transfered and the damage they inflict is still the same. So switching to heavier BB's to fool the chrono is cheating and is grounds for immediate ejection from a game.


VELOCITY REDUCERS DON'T WORK
http://evike.com/forum/viewforum.php?f=20l
These are accessories that look like a mini-suppressor and replaces your flash hider at the end of your barrel. Using a series of flanges they hold back the outgoing pressure of the barrel and slow down your BB (though I have heard a competing theory that they reduce the BB speed by simple friction).

Unfortunately because they are easy to put on, they are just as easy to remove which is why most field's don't allow them to be used at chrono. It's all too easy for a dishonorable player to take them off after chrono and use a hot gun on the field.

But the larger problem is that while they do reduce the speed of a single BB, they do not work in full-auto because the flange is open and every other BB after the first is no longer reduced. Thus velocity reducers are not a real solution to downgrading your FPS.


TURNING UP YOUR HOP-UP
Hop-ups work by forcing a BB to squeeze past a rubber (or metal) bump at the top of the barrel and imparting backspin to the BB. But squeezing past the hop-up bucking also slows down the BB. Thus you can lower your FPS by simply increasing your hop-up setting. But like Velocity Reducers this is not a permanent solution and can be considered 'cheating'. So check to make sure their hop-ups are turned down.


DIRT
Intentionally 'dirtying' your barrel with dirt, dust, paint, etc. can introduce enough friction in your barrel to lower your FPS but can cause jams too. Not a good idea.