Open Muzzle Swap and Upgrade

It’s amazing how easy it is to create something bad and how difficult it is to create something good. That’s why this is going to sound a bit tricky, but in the end, it’s all worth the time, money and effort.

For this demo. I will be using a stock JBL Mini Magnum. A customer pulled it out of storage and said to “fix it up!” I love when people tell me that!!! For many of you, myself included, you may ask yourself why would I want to pay more for upgrades than the gun was worth itself, even brand new? Simple! When upgraded, it’s like a COMPLETELY new and different speargun. The Mini Magnum acts like a .50 caliber sniper riffle. Well, not exactly, but it’s hard to believe how nice these upgrades make out of this relic until you try it… All these upgrades / fix-ups apply to all Euro guns as well. Some exceptions may apply.

1. The stock JBL muzzle has so many issues I don’t even know where to begin. So I did WHATEVER it took to get the damn thing out of the aluminum barrel without damaging the barrel. This may well be the most difficult part of the whole upgrade / install depending on the condition of the gun.
2. When the original muzzle is off, I gently placed the new open muzzle into the barrel. The open muzzle is a smidgen too big so I gently and evenly buffed down the circular part of the muzzle, the part that fits inside the barrel. Note: I was careful not to create an uneven circle, which may cause the muzzle to rock or move around when installed into the JBL barrel. Note: I was careful not to shave too much off of the circumference of the muzzle. This also may cause rocking or moving around when installed into the JBL barrel.
3. I made sure the length of the muzzle itself passes far enough beyond the existing hole in the barrel that I will have enough material of the new muzzle to drill into. When I was satisfied with the snug fit between the open muzzle and the JBL aluminum barrel I began preparing the epoxy. A five-minute epoxy works best. Once everything is prepared properly it is helpful to have fast setting epoxy so I am not stuck waiting for it to dry before proceeding forward.
4. I applied epoxy to the inside of the barrel. I DID NOT apply epoxy directly onto the muzzle for this application. The result will be oozing epoxy everywhere out the front of the barrel rather than where it needs it – inside the barrel.
5. I placed the open muzzle inside the barrel until it sat flush. I placed a STRAIGHT shaft into the JBL mechanism. I held the speargun up to my eye, horizontal, being careful not to let the shaft dangle or fall because there’s nothing holding the shaft anymore. I “Eyeballed” the muzzle as best I could, using the spear shaft as my guide to align it with the mechanism and handle. I adjusted the muzzle accordingly. Once straight, I allowed the epoxy to fully dry.
6. The epoxy serves several purposes, but a “pin” should be added to the muzzle assembly for extra strength and peace of mind. I don’t want to rely solely on the epoxy to hold my prize fish once it’s speared. A hole needs to be drilled through the now dry and set epoxied muzzle to set the SS pin. I can’t emphasize enough how CAREFUL I drilled the hole. I made sure the original hole on the side of the JBL barrel AND everything else is aligned properly so the drill bit ends up coming out the other hole in the JBL muzzle… not creating a “new” hole.

   Note: The chosen SS pin should have a large enough diameter to fill the existing hole(s) in the JBL barrel, and long enough to stick out slightly on each side of the barrel. I use a SS pin milled to a close tolerance as to allow enough of the pin to add “pull strength” to the muzzle but not to stick out too much to catch on anything and / or just look kind’a corny.

   The drill-bit diameter needs to be a smidgen smaller than the SS pin I am using. I found just the right diameter drill bit to require a light tap with a small hammer to add the pin into the hole.

   I added a very small amount of epoxy to the inside of the newly drilled pinhole than placed the pin through and aligned it properly. Lastly, I wiped off any excess epoxy and let the epoxy set.

Track Install

JBL makes a great poly track but it needs a little help to fit each specific application. First, all the tracks come in one cut-to-fit length. Now that my open muzzle installation is finished and the epoxy is dry, I can set the full-length poly track in place to measure the length and trim it.

1. I don’t want to make this too complicated because I know it doesn’t need to be perfect, I just don’t want to make it too long. Slightly short is ok. I carefully finish the cut end of the tract to remove burs, unevenness or anything that would cause the track not to sit flush to the barrel. Once the track is cut and finished and the tape protecting the sticky-bottom is still in place, I placed the cut piece back onto the JBL barrel to check fit.
2. Next, I wiped off the surface of the barrel with isopropyl alcohol to make sure it is clean of dirt and oil and allowed the surface to dry.
3. I used a new or perfectly straight shaft to tie to the muzzle while the other end of the shaft is loaded into the mechanism. I use this application to set the track straight and even.
4. I peeled off the protection tape from the bottom of the track, mixed and applied a very thin, smooth and even layer of epoxy to the track…and did this quickly. If too much epoxy is used I would have an incredible mess to clean up once the track is pressed into place. I took into account that the fast drying epoxy may set by the time I finish preparing the track from one end to the other so I moved quickly.
5. 5. I quickly and carefully placed the track into position under the shaft. I raised the gun horizontally and pointed it into a brightly lit area. I use the bright light to highlight any inconsistencies to get a perfectly straight track. I would lower and check the application from directly above insuring the shaft, which is my guide, is still in proper alignment from the muzzle to the mechanism. I checked very, very carefully from the gun's horizontal, eye level orientation and alternate looking from straight above the gun until the track is perfect BEFORE I applied any direct or downward pressure to the track. Once perfection is achieved, I pressed the shaft onto the track gently about every three to five inches, at first, from the mechanism toward the muzzle. I alternated removing the shaft and placing it back into position to get a perspective of the track with and without disruption. Next, I checked and rechecked the straightness of the track as I applied more and more pressure to the shaft against the track to permanently secure the track to the muzzle. I repeated this process carefully until the epoxy was dry or until I was sure everything was perfect. Lastly, I set everything aside to cure.

Custom Shaft

A custom shaft will complement all the changes I’ve made this far. For this application I will use a 1/4” Riffe brand spear shaft. The shape of Riffe’s and JBL’s sear is similar enough that with some simple modifications to the Riffe shaft it will work great with all the modifications on this application. The new muzzle will allow us to use the Riffe’s hardened, stainless, spring steel spear shaft with a raised tab for use with Dacron wishbone rubber bands for silence, strength and safety.

1. The tab nearest to the back end of the Riffe shaft needs to be carefully ground off to allow the shaft enough length to fit into the JBL mechanism. I was careful not to grind or cut off too much material, which can weaken an already thin 1/4” shaft.
2. I also removed the barb/flopper (in the front) so I could evenly polish the entire shaft. My intention was also to replace the short Riffe barb/flopper with a longer generic Euro barb before I was complete with this project.
3. When applying the Euro barb I was careful not to over-smash the rivet. However, a small adjustment with a small flat-head screwdriver is common to allow the barb to open properly once in place.
4. I am done with a barb / flopper type shaft when the barb opens only ever-so-slightly at rest. Once a fish is speared and fights the slightest amount, the barb will open completely and stick open. This effect is achieved by gently tapping, at first, the topside of the barb to slightly crush the barb inward creating friction between the barb and shaft. This affect is very helpful but only lasts the first few fish. Better than nothing, because it’s the small things that makes the all the difference at the most important times.

I finished the project by adding a lightweight bungee swivel and 200 pound black mono to keep the lightweight theme and to maximize range and accuracy. For an appropriate amount of power and stealth for this application I chose two black (2) 9/16 X 20” high modulus Riffe Rubber bands. Why did I do all this??? Cuz can!