This is a quick post about some common mistakes that can occur if you aren't careful when laying out a muzzle on a wood rollergun. Ask me how I know! Depending on the hardware you decide to use some of what I will talk about here may not apply so keep that in mind.
To date I have built two double rollers five single rollers three roller hole guns and one inverted rollergun. Of those eleven guns the ones that performed the best all had open tracks. I'm not sure why but the open track guns all shot more predictably with better accuracy and with a flatter shaft trajectory. These guns all had a "deep open track."
Parts I use at the muzzle on an open track rollergun: Delrin rollers; sometimes with ball bearings, sometimes without. I haven't noticed a difference performance-wise. A muzzle riser similar to this one.
I have used kits similar to this one but I'm not really a fan.
(They work fine, just not my cup of tea)
The first mistake that's easy to make
You can very easily drill the hole for the pin that holds your rollers in place through your track! Be careful and draw a line on the side of your blank that indicates the depth of the track you milled. Make sure you drill this hole below the track when the time comes. For me that time comes before milling out the cavities for the rollers themselves. If you drill the hole after milling out the roller cavities you will have three (instead of one) instances where there will be the possibility of wood tearing out as the drill bit exits the hole. When drilling any hole through your blank use a pice of masking tape on the outside of the blank to minimize or prevent altogether ugly tearout that might occur. The outside part of the blank that the drill bit exits through should also be backed up with another piece of wood.
Keep in mind, you want to get the rollers at a height that keeps your power bands from dragging or pinching on the wood of your speargun. Because of this try and get that hole as close to the track as possible without going through the track!
Above photo shows two blanks on their way to becoming rollerguns. Muzzle Riser Pitfalls:
Things to consider when laying out the position of the muzzle riser. First, that pesky pin that the rollers ride on. Most muzzle risers are held in place with screws. Make sure you place the riser in a position that will have it's screws AWAY from the pin sitting directly beneath it.
This photo illustrates how to position the riser so you don't run into problems with screws hitting into the roller pin.
Speaking of the screws that hold the muzzle riser in place don't forget to leave enough wood for them to screw into as you are shaping the muzzle. 
One last thing
Be sure the muzzle riser will sit slightly below the track to prevent the possibility of anything weird happening as the shaft exits the gun. I use a half inch straight bit to mill out the slot for the muzzle riser to a depth that will keep it slightly below or flush with the depth of the track. Hopefully you can see this in the photo below.
I hope this short post helps you on your speargun building journey. Feel free to comment or eMail me with any questions or comments you might have.
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