FPS Dramatic Increase with BB Weight

[The Dr]

Hi All

Just wondered if someone could shed some light on this.

I went to a site on Sunday who chronod my gun on both 0.20's & 0.40's.
On .2's my L96 was firing at 498 but on 0.4's it was firing at 396.
This was deemed too hot and i wasn't allowed to use the rifle.

I was just wondering why my fps "increased" by around 60 by using a heavier BB.
I say increased because they were using one of the conversion charts and 396 fps 0.4's would be the same as 560 on 0.2's.

Cheers
Ric

[Smintheus]

Re: FPS Dramatic Increase with BB Weight

Gas or spring rifle, bud?

Having just done my own calculations to check their sheet :

0.2 498 FPS ~ 351 FPS on .4 = 2.29 J

0.4 396 FPS ~ 560 FPS on .2 = 2.9 J.

So there is a significant muzzle energy hike going on between the two.

[HuntR]

Re: FPS Dramatic Increase with BB Weight

It's normal for a gas rifle, as the pressure or C02/Green Gas building up behind the heavier bb. I can't say i've ever witnessed this with a spring rifle though

[Figjam]

Re: FPS Dramatic Increase with BB Weight

I've never seen it with a spring gun. The theoretical reason is because the heavier BB spends more time confined in the barrel with compressed gas behind it, thus imparting more energy, but its more a thing in gas guns because of the way propane decompresses. Physics boys will be able to give you some maths.

Its an exceptionally thorough site to be chrono-ing with the weight of BB you planned to use. Would they have let you play if you'd promised to only use .2s?

[Macleod]

Re: FPS Dramatic Increase with BB Weight

The most logical reason seems to be that the BB's your using might not be .40s
What brand of .40's did you use?

[Figjam]

Re: FPS Dramatic Increase with BB Weight

The most logical reason seems to be that the BB's your using might not be .40s
What brand of .40's did you use?

Nuts, hadn't thought of that!

[The Dr]

Re: FPS Dramatic Increase with BB Weight

Cheers for the replies all.

Mine is a spring rifle, as it is a laylax/pdi/well L96.

I am using guarder .4's

I was quite annoyed that that is the only site that wouldn't let me use my sniper.

Do all sites chrono with the actually weight you use or with .2's? I would like it so I can use my rifle at all site.

Btw the spring i use is the 500fps spring (can't remember name) but it has been in the rifle for over 2 years so should be less by now.

Cheers

[jonny lovegrove]

FPS Dramatic Increase with BB Weight

Sites chrono on their own 0.2g bbs so that they can confirm that is ACTUALLY the weight in the mag.
Gas expansion wont be the problem as air isnt compressed in a spring rifle to any meaningful PSI. I would echo the weight call. Would be about right for 0.36g bbs. Weigh one?
Edit: or count out 10 and weigh 10 ect

[Smintheus]

Re: FPS Dramatic Increase with BB Weight

Firstly, check things like airseal (shot consistency), and clean barrel. Beyond that, I would suggest that as it's a spring rifle, the most likely explanation is brand of BB.

Obviously all brands are close to 6mm, but some may be based around 5.95mm rather than 6. If your .4s are a different brand to your .2s, that might be causing it. Are they measuring on a few shots to account for variance, or just a single shot at a time?