Well I finally got a chance to revisit the .35 cal SPA M25. It's been mothballed for a while....when I set it aside, it was still producing poor groups after having taken a first pass at correcting the poor factory crown and working the bore with a cast lap. That history is covered in this 2023 thread if anyone is interested:
My lingering suspicions were with the threaded stub at the muzzle:
Pushing through a pellet, I could feel it relax a little right at the end, which is a pretty typical byproduct of removing material from the OD for the threading operation.
Some the available options to deal with this problem are:
And then on to making an adapter that fits over the end of the barrel and provides the threaded feature for attaching the shroud and LDC. A few operations into turning it from a piece of 1" aluminum rod stock:
For a novice like me, single point threading on a manual lathe is a bit nerve-wracking but I like this method. Mount the cutter upside down and run the lathe in reverse so the carriage is autofeeding away from the workpiece. Easy peasy.
Here's the end result.
The attractive machined finish is gone because I did some final shaping on the OD with an emery wheel and files. Why? Well, I wanted to use grub screws to secure it to the barrel, which meant I needed the wall to be thick enough to thread...but that meant it would barely fit through the shroud, blocking rearward airflow and rendering the shroud ineffective. So what I did was thin out the wall everywhere except in four little anthills where each of the grubscrews is located. It ain't pretty but it works, and thankfully no one but me will probably ever see it.
Back to the barrel itself. In the lathe, faced off the end to get rid of the rough hacksaw cut. Then applied a crown with the brass screw method:
I don't care how many times I see it, there's just something satisfying about a crisp crown.
Then poured a cast lap:
And proceeded to work the bore:
Even with a fairly aggressive abrasive, the work goes slowly. A generously long lap is good for maximizing the scrubbing action:
While I had it apart, I looked over at the hammer spring and decided to see about addressing the annoying hammer bounce with a simple SSG. Started by taking a piece of scrap stock and making a spring retainer. Simple stepped part dimensioned to the spring ID and OD, center drilled and tapped. Parting it off in the lathe:
Threaded a piece of 4mm brass bar stock on each end and drilled a clearance hole in the end cap. The parts laid out:
Dry fitted together:
Back into the gun:
Then making room for it in the stock. Took a 24" long 1/4" drill bit and bored out a hole for the SSG, followed by some hand shaping a bell mouth with a carbide burr:
Again, zero points for pretty.
Then some trial and error getting the preload tweaked for the velocity knee:
Ahhh, the flatulent hammer bounce is gone. But the burning question is, will it group now or will it continue to make buckshot patterns? Well, here's the first 50yd 5-shot group:
I mean, it's not a great group but it's the best group I've gotten from this rifle to date (0.88" at 50y).
Could just be a fluke but I'm hand pumping this beast so I decided to try one with the JSB domes:
That one measures 0.56" CTC...flirting with MoA. I'm definitely going to have to do more testing but these early indicators suggest a barrel of a very different character.
That's about it for now. My hope is this pictorial will provide at least a teaspoon of inspiration to someone to tackle a stubborn barrel and whip it into shape. This was a long post so thanks for reading.
My lingering suspicions were with the threaded stub at the muzzle:
Pushing through a pellet, I could feel it relax a little right at the end, which is a pretty typical byproduct of removing material from the OD for the threading operation.
Some the available options to deal with this problem are:
- apply a choke - I just haven't had much success with this in the past.
- lap the bore - it's one thing to pour a cast lap and work out a tight spot, but when the tight spot is 98% of the length of the barrel, no thanks!
- chop off the offending threaded portion - this approach is the easist solution, except that I want to retain the factory shroud and LDC.
- replace the barrel - probably the best option in terms of odds of success, but I'm just not inclined to spend that kind of money and machining time, at least not until I've tried addressing the known deficiency of the OEM barrel.
And then on to making an adapter that fits over the end of the barrel and provides the threaded feature for attaching the shroud and LDC. A few operations into turning it from a piece of 1" aluminum rod stock:
For a novice like me, single point threading on a manual lathe is a bit nerve-wracking but I like this method. Mount the cutter upside down and run the lathe in reverse so the carriage is autofeeding away from the workpiece. Easy peasy.
Here's the end result.
The attractive machined finish is gone because I did some final shaping on the OD with an emery wheel and files. Why? Well, I wanted to use grub screws to secure it to the barrel, which meant I needed the wall to be thick enough to thread...but that meant it would barely fit through the shroud, blocking rearward airflow and rendering the shroud ineffective. So what I did was thin out the wall everywhere except in four little anthills where each of the grubscrews is located. It ain't pretty but it works, and thankfully no one but me will probably ever see it.
Back to the barrel itself. In the lathe, faced off the end to get rid of the rough hacksaw cut. Then applied a crown with the brass screw method:
I don't care how many times I see it, there's just something satisfying about a crisp crown.
Then poured a cast lap:
And proceeded to work the bore:
Even with a fairly aggressive abrasive, the work goes slowly. A generously long lap is good for maximizing the scrubbing action:
While I had it apart, I looked over at the hammer spring and decided to see about addressing the annoying hammer bounce with a simple SSG. Started by taking a piece of scrap stock and making a spring retainer. Simple stepped part dimensioned to the spring ID and OD, center drilled and tapped. Parting it off in the lathe:
Threaded a piece of 4mm brass bar stock on each end and drilled a clearance hole in the end cap. The parts laid out:
Dry fitted together:
Back into the gun:
Then making room for it in the stock. Took a 24" long 1/4" drill bit and bored out a hole for the SSG, followed by some hand shaping a bell mouth with a carbide burr:
Again, zero points for pretty.
Then some trial and error getting the preload tweaked for the velocity knee:
Ahhh, the flatulent hammer bounce is gone. But the burning question is, will it group now or will it continue to make buckshot patterns? Well, here's the first 50yd 5-shot group:
I mean, it's not a great group but it's the best group I've gotten from this rifle to date (0.88" at 50y).
Could just be a fluke but I'm hand pumping this beast so I decided to try one with the JSB domes:
That one measures 0.56" CTC...flirting with MoA. I'm definitely going to have to do more testing but these early indicators suggest a barrel of a very different character.
That's about it for now. My hope is this pictorial will provide at least a teaspoon of inspiration to someone to tackle a stubborn barrel and whip it into shape. This was a long post so thanks for reading.
