Attempting to rescue a bad barrel - SPA M25 in .35 cal

[nervoustrigger]

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:

• 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 easiest 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.

So out came the hacksaw...


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 grub screws 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.

[bear air]

Nice work Jason and thank you for sharing.

[avator]

Thanks for the insight Jason.

[anti-squirrel]

Great little tutorial/process, Jason. 

We didn't have to monkey with our M25- it shoots almost as accurately as Sonja's Lelya at 50 yards.  Still, I enjoy following your projects fixes.  You really woke that barrel up, and with accuracy like that you have a meal-maker.

[nervoustrigger]

Thanks guys, I do appreciate your kind words of encouragement.

I’m hoping to have some time this weekend to look into making a PEEK poppet and MDS nylon hammer.  Even though it’s a >100fpe big bore, the recoil and muzzle report seems excessive so hopefully I can make it more enjoyable to shoot.  My guess is I’ll quickly find a 100% plastic hammer will not be adequate, and I’ll need to switch to a hybrid metal & plastic for mass, but I intend to start with plastic and go from there.

[sb327]

Nice write up, I enjoyed it. Keep us updated on the hammer and poppet.

Dave

[bear air]

Nice work Jason. I'm looking forward to seeing your results.

[nervoustrigger]

Okay, well it has been a lovely long weekend with a good blend of family and shop time.

I'll open by saying I did not follow my original plan of trying a plain MDS nylon hammer.  But I'll get back to that in a moment.  First I'd like to touch on the barrel porting.  The factory porting consists of a groove with 2 drilled holes:


Since the barrel threads into the receiver, this arrangement is good for manufacturability because the factory does not have to be concerned with where the ports end up in relation to the valve's exhaust.  In my particular case, the two ports unfortunately ended up at 3 and 9 o'clock, maximizing the distance and sharp turns the air must take to reach the pellet.  So I drilled a 3rd hole in line with the valve and tried running it like that for a few days, but I knew the better approach would be to fill the groove with epoxy and mill a single large-ish port in line with the valve's exhaust:


Without touching the hammer spring tension, this change yielded a 6% increase (110fpe -> 117fpe), in spite of the fact the new single port is smaller than the combined area of the 3 round ports.

Now back to the lightweight hammer and PEEK poppet.  So why did I decide against trying a 100% plastic hammer?  Well, I removed the OEM hammer and put it on the scale:


Yikes.  No way going from a Delrin poppet to PEEK is going to make it that easy to knock open.  So I decided to just go ahead and make a 2-piece hammmer with a steel core and MDS nylon body, and target a weight somewhere in he ballpark of a third of the original. 

And, oh look, an offcut from a .177 Crosman barrel weighs about 24 grams:


Drilling and turning the nylon body:


The finished 2-piece hammer weighs 33.6 grams, or about 31% of the OEM hammer:


Then I turned a new poppet and selected a weaker valve spring (bottom) to replace the originals (top):


Then I reassembled and tried test firing it with no changes to the hammer spring preload.  The new velocity?

0 fps.  Haha, it barely blipped the valve, the pellet didn't move, and I could hear just a tiny psssssssshhh of air bleed out of the breech over the course of a few seconds.  Okay I'm going to need some more hammer spring so I rummaged through my Cool Whip container of springs.  Every spring even remotely close to the right gauge and diameter was too short, but I'm too impatient to wait to get some springs in so I picked out a couple and turned a little plastic coupler to join them end to end:


Reassembled into the SSG:


Ah, there we go:


258mps (846fps) with an 82.2gr pellet for 130.6fpe.  That's up on the velocity plateau so I need to back it off a little, but even with it wasting air, the muzzle report is already noticeably less boomy so I'm happy with how it's going.

[bear air]

That's some darned good work Jason. Keep it up. Your really doing some incredible things with this rifle.

[anti-squirrel]

Excellent work, Jason.  It kinda showcases that the SPA airguns are prime for tuning with some know-how.  I remember reading a few threads about tuning and waking up the M-series bullpups


I don't need to bring up how many people have massaged their PP700 pistols.  Mine responding very well to tuning hammer spring tension, orifice, and regulator.