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Bite The Bullet

Myth Busters: Do Longer Barrels and Heavier BBs increase Energy? (2009-02-27)

It is common knowledge that changing your bb weight is a way to change your gun's FPS but this is more pronounced is gas weapons. Because the gas in the barrel is expanding throughout the entire firing cycle the bb is accelerating for the entire length of the barrel. This means that the change in velocity from having a longer barrel is much more pronounced than on an AEG where the spring and piston provide a more constant push that reaches it's terminal velocity much faster.

To show the effects of different barrel lengths on the FPS change caused by changing bb weights, we use two G&P Western Arms Official Custom gas blowback armalite based guns. The G&P WOC SPR, and The G&P WOC Tank.

The Barrel length on the SPR does not actually extend all the way to the end for the reasons described above; to keep the power down to a reasonable and skirmishable level. As shown in the picture, the inner barrel ends abruptly at 230 mm down the length of the outer barrel, which is to say about half the length of the outer barrel.


SPR Barrel

In the G&P WOC Tank, the barrel extends all the way to the very tip of the inside of the flash-hider and can be seen by looking into the barrel at an angle (don't look directly into any barrel, EVER). It is 170mm in length.

Tank Barrel

Each gun was chronoed 5 times with each weight of bb tested. 0.12g, 0.20g and 0.25g bbs. then the average was taken as the FPS for that weight and gun. The same magazine was used for both guns to keep the test as accurate and reliable as possible. The temperature during the test was 27.3°C and the relative humidity was 90%.

  170mm 'Tank' 230mm SPR
0.12g BBs 399fps 420fps
  395fps 424fps
  402fps 419fps
  395fps 425fps
  401fps 427fps
Average: 398fps 423fps
0.20g BBs 341fps 373fps
  344fps 373fps
  339fps 375fps
  339fps 372fps
  344fps 372fps
Average 341fps 373fps
0.25g BBs 324fps 357fps
  324fps 341fps
  322fps 347fps
  324fps 357fps
  330fps 344fps
  323fps 349fps
Average 324fps 349fps

Next the FPS was converted to Joules, using the formula K=½(mv²). Or using a convenient converter like the one found in

  170mm 'Tank' 230mm SPR
0.12g BBs 0.88 1.00
0.20g BBs 1.08 1.29
0.25g BBs 1.21 1.42

On the G&P WOC 'Tank' with a 170mm barrel, the lighter BBs caused BB energy to drop by 19% due to it spending less time in the barrel. while on the G&P WOC SPR with a 230mm barrel, it dropped 22%. This just goes to show that changing the BB wight changes the overall muzzle energy quite drastically by lightening the BB. So for those of us skirmishing in CQB conditions, we suggest only using 0.20g BBs to avoid injury.

Using heavier BBs caused the 'Tank's power to rise by 12% and the SPR's power to rise by 10%. Due to the bb staying in the barrel longer and therefore accelerating and building up energy longer.

Using the information will hopefully let players better understand the physics behind gas blowback gun velocity and power and use this information to skirmish safely.