As I said in the photo thread, this is a SNAP, but with very different internals. Let's explode that sumbitch:
And now: the ridiculously photo-heavy writeup.
The idea for this blaster came about after MANO last September. I had just finished the SNAP-8, my take on an internally sprung PAC-style blaster. Aside from mechanical issues, I just ended up not liking it. What keeps drawing me back to the PAC is how smooth the priming action is, with almost no friction. Compression spring systems have far more friction points, so the priming action isn't as smooth. So I decided that I needed to build a blaster using extension springs. (For those keeping track, yeah, I never posted the SNAP-8. There's a reason for that.)
Rather than run the spring external to the plunger tube, I decided to run the spring down the middle, with a coupler sanded to the traditional ramp and catchface. This was mostly due to the length of an extension spring system. It didn't take me much figuring before the benefits became pretty clear:
• Greatly reduced plunger friction means a weaker spring can be used. Easier to prime, equal performance.
• Safety: when the system is primed, the spring is trying to pull the blaster together, rather than blow it apart.
• Making a dry-fireable system would be trivial, as the spring itself limits the range of travel.
• Having the spring inside the plunger tube means no more snagging the nail on the spring.
• The nail rests on the plunger tube for the entire action of priming. Because of this, it doesn't matter how long the nail is. Goodbye, fussy nail trimming.
The design of this blaster ended up with another benefit: it can be almost completely torn down without tools. Future updates to this design will be able to be completely taken apart without tools. It should also be pretty simple to make this dry-fireable, as the spring itself limits travel. It pretty much is dry-firable right now, as the plunger head doesn't impact the front bushing.
So, let's look at the parts a little closer:
This is a general list, as this isn't a step-by-step writeup. Your mileage may vary.
1.25" Sched 40 PVC
1.25" -> 1.5" bushing
1" -> .5" bushing
2" Sched 40 PVC
.75" thinwall PVC
.75" Sched 40 PVC
Threaded rod, or any manner of sturdy metal
9.75" x .5", .062 extension spring (available at Menards)
Basic SNAP parts:
Handle (this handle is my basic firring strip/PVC clip method)
Endcap plunger seal
This is pretty much standard SNAP: basic CPT, usual handle. I cut the slot all the way through the back for ease of assembly (decided upon during the build process). We'll get into how this is put together later on. I chose an arbitrary draw length of around 6" (which is reasonable for my ~10" extension spring).
Here's what makes it different. The catchface, instead of being right by the plunger head or at the end of the rod, is situated in the middle. Instead of an endcap, it's a standard 3/4" coupler. I took a few minutes to slope the rear of it to create the ramp for the nail. Benefit of this design: the ramp can be very gradual, making for smooth priming action. Also, the nail itself is limited in its travel by the rear of the plunger tube. In other words, there is no nail "tuning" needed for this style of SNAP. A too long nail which wouldn't work in a standard SNAP will just rest on the rear of the plunger tube, and glide until the catch.
The rear of the plunger is .75" sched 40 PVC. The front is .75" thinwall, for easy slot cutting and light weight. The plunger head doesn't impact a bushing, so structural integrity is't really needed. The slot os also not for any accuracy, it's more for "get the hell out of the way of the spring".
The plunger needs some manner of securing, to keep it from pulling apart during priming (thinwall doesn't hold in a coupler that securely, and grinding off the end of the coupler means the sched 40 PVC doesn't hold that well, either. I'm currently using two small screws. The hole in the coupler is wide enough to let the head all the way through, and it screws into the tube. It's enough to pin the tube together. I'm going to use adhesive on later builds.
The spring is from Menards. 9.75"x.5", .062 wire. The spring at rest sits in the solid tubing on the right, pinned by the thicker bolt through the endcap. The thinner bolt holds the loop at the other end. The slotted length allows the plunger to move back while the spring increases in length.
I did a post-war disassembly to check on the durability of this design. As it stands right now, it seems that reinforcement won't be necessary. This follows, as the spring in this blaster is a little weaker than the springs I used for a basic SNAP-1, which never needed reinforcement.
Yeah, it's a tube. The cuts allow it to be snapped on to the front spring rest (the thinner bolt). I need to work on this section a little, as it can occasionally wiggle its way off. Ideally, this part should also be under spring tension, so the pull of the spring itself can hold the entire blaster together.
What you'd expect. It's actually a couple pieces taped together, because I built this gun out of scraps. The top slot doesn't need to be as long as it is, but it helps keep weight down a little. The bottom slot is a little wide, so it's being guided by the screws that hold the handle on (screws will be replaced by epoxy, and strips of PVC for guides.) There's a sanded out ring of 1.25" coupler in front to keep the thing from wobbling. It just pushes the bolt in the rear, pushing back the plunger.
Putting it together:
1. Drop the spring into the plunger tube and pin it with the large bolt. When all is said and done, you'll want to keep the spring centered on the bolt.
2. Slide the plunger into the receiver.
3. The spring is under a bit of preload, so reach in with a screwdriver and lift it up. Hold it in place with the screwdriver. Then, slide the bolt under it and slide out the screwdriver.
4. Pop on the plunger tube.
And stick the rubber washer endcap on the end. The seal style I'm using is identical to the SNAP-1.
5. To allow the Tee for the stock to slide on, I used a 1"->.5" bushing wrapped in tape, then shoved on the tee.
I'm very pleased with how this blaster turned out. I used it the entire day on Saturday, and had no failures. The spring popped loose once when someone else dry fired it repeatedly, but restringing it was easy to do in the field. Ranges were comparable to any pump action SNAP on the field, with an easy priming action. Refinements are needed, but I have my new go-to primary.
Things to do:
• Reinforce the plunger catch
Yeah, I said it didn't need it, but I want to make this thing bombroof. I'll test out JSB's suggestion, as well.
• Attach the handle with epoxy
Currently, there are screws holding it on that have to be backed out a bit to remove the plunger. I'm going to trim those off and use epoxy to permanently attach the handle (and possible add a better, slightly more shapely handle).
• Fix the plunger tube attachment method.
It has a bad habit of wiggling off. I want to find a method that will hold it securely, but not use screws.
Edited by Carbon, 23 February 2015 - 09:39 AM.