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Old 02-22-2007, 05:22 PM   #1 (permalink)
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Size of the ball bearings in a Speed Demon bolt - detent & velocity adjuster

As the title says, I'd like to know what size the ball bearings are in a Speed Demon bolt?

The hole that holds them looks to be bigger than the one in the PGP bolt so I suspect they are a different size but I could be wrong.

Last edited by Surestick; 04-25-2007 at 10:11 AM. Reason: added info in a new post - changed title to reflect info contained in the thread
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Old 02-22-2007, 06:39 PM   #2 (permalink)
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1/4" i think.
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Old 04-25-2007, 10:09 AM   #3 (permalink)
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Ok, I've done some figuring out:

1/4" BBs are just too big & the next size down, 7/32", is too small (loose enough that they either fall out through the holes in the side of the barrel or one of them falls down in front of the rod the is supposed to lock them in place & blocks it).

After doing some math & a bit of time spend w/some calipers it looks like 6mm BBs should be about right.

For posterity, and because I couldn't find the info anywhere else on MCB or the net, here is the info I have collected on teh Speed-Demon bolt.

Dia of the BBs for velocity adjuster: 5/32".
You need two, one on each side. With only one (or none) there is a path for gas to escape out the back of the bolt & you will either have trouble hitting a reasonable velocity or lose efficiency.
Dia of the BBs for the detents: 6mm (I still need to test this though).


Dia of the hole in the bolt that holds the detent BBs: 0.248 inch = 6.30mm
Dia of the holes in the barrel that the detent BBs lock into: 0.219 inch = 5.56mm

1/4" = 0.25" = 6.35mm so 0.02" too big for the bolt and when I tried they they don't fit so...
7/32 " = 0.219" = 5.56mm & these fall out through the holes in the side of the barrel.
6mm should have 0.3mm clearance in the holes in the bolt & be 0.46mm too big to fall out the holes in the barrel.

I wonder why Sheridan went metric for one piece?
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