Author Topic: Modifications Bullpup PCPs.  (Read 1516 times)

Offline Novagun

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Modifications Bullpup PCPs.
« on: July 12, 2013, 09:23:31 PM »
    Hamerli AR20FT Bullpup Project. 

The Hamerli AR20FT rifle is a very capable rifle out of the box and very accurate but sensitive to how the shooter holds it and on occassion this leads to a point of impact shift. The owner had been customising the rifle to suit his needs.
 The rifle had a plastic stock that tended to flex. The owner who is not a highly skilled woodworker decided to make a laminated wooden stock and change the configuration of the rifle.


The rifle was stripped down so a design for a new stock could drawn up.

First a mock up Bullpup stock first using a couple of pieces of Ply screwed together with wooden blocks  as spacers to allow mounting options and the remote trigger system to be developed. This mock up showed up some deficiencies that were later remedied.


The builder will now explain how he went about the project.

From Construction Ply I cut 4 identical sized pieces that were large enough to accomodate my template. In fact I made two templates, one that had the shape of the final product and a second that was cut to allow for the action of the rifle and necessary inletting to get it to fit. The remaining two Layers, one on each side would hide all of the Inletting.
One of the toughest technical challenges I could forsee with this stage of stock building was going to be drilling the Holes that would attach the stock to the rifle, they need to be perfectly central, straight and had to not only allow for the bolts but also the tools required to tighten them. I decided to overcome these challenges by channelling the two central sections of ply using a router, instead of trying to drill holes after the ply layers were laminated. For the font bolt that passes through the centre of the handgrip I cut a 6mm groove on each of the two middle layers, forming a 12mm hole for my Bolt, I then cut a 20mm slot in each piece to allow me to get a socket and nut to the Bolt from underneath once the stock was complete.
The Rear mount is a simple 6mm Allen Bolt that passes through the rear of the action like the original stock, and I allowed a 10mm lead in to access the Bolt from below.

   Next I cut the two external layers that would hide all of the inletting done so far. I left ample spare timber at the front and rear of the project and used short square drive screws to dry laminate the entire assembly. This allowed me to trim  and sand the edges of the stock before Lamination.
 Once this was ready and the Action of the Rifle had been dropped in place to check all clearances and operation of a mock up trigger linkage, I glued  together all four layers using the screws at the front and rear of the stock to help align everything and clamped it all to dry.
When the glue was set  I cut the Front and Rear ends of the project to my template. This  removed the laminated sections that had the screws still in them.
There were a couple of areas on the stock that were critical to not cut too deep during the shaping process around the bolt and action inletting as they were only  12mm thick. To make a guide for these areas I ran a 5mm  router cut and knew not to go past that mark. I also marked on the outside of the stock the exact location or all inletting and bolt holes so I had a constant reminder.
I also drilled three starter holes for the section of the stock that was to be removed behind the handgrip.

Next step was to set the Router to work to do some of the rudimentary shaping of the stock.
This project was my first time using a router but by setting up of jigs, and that proper planning the task was a very quick and easy achievement but it can not be rushed.!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!111


Now it was time to clean up some of those lines, and start searching out those cool patterns I had seen on professionally built laminated stocks.
I used a Dremel with some sanding wheels and started the shaping. You have to keep it moving or it will start to dig holes, but it allowed for relatively easy shaping and access to some of the tighter nooks and crannies with ease.
Revealing the different grains in the different layers of the Ply was both a pleasant surprise and mind racking at the same time, a grain can just start to present itself, and next thing you know your through it and into the Epoxy used to bind the sheets. Shaping these Layers to flow with the overall design of the stock was difficult, it takes time and patience, and also the will power to overcome the use of the power tools and pick up the good old sanding block and 60 grit sand paper.
After this I used 180 Grit and started the final stretch of shaping and cleanup. Another point with Ply is that not all Layers are laid with the grain running in the same direction, this means that at some point you will have to decide which way you are going to sand, and be aware that the softer areas between layers of grain will sand away quicker than the hard.

After many Hours of Sanding over a few days I needed a change so decided to set about the cheek rest.  I knew it was going to need to be mounted on the scope rail of the rifle  but I had yet to determine exactly how I was going to mount it. I started by laminating together three strips of leftover 12mm scrap and set that aside to dry overnight.

Next I played around with some old dovetail scope mounts, the idea being that I would recycle the lower dovetail section for my project. Then  I was sifting through my box of miscellaneous hardware and found some left over odds and ends from some form of kitchen cabinetry. Here I found a part that looked very similar to a dovetail mount, even with a channel out of it that was just the right diameter for the slimmer section of the dovetail rail, so with a triangle file I made it fit over the 11mm dovetail rail.

 The next day it was time to get to work shaping the cheekrest, but first things first - Inletting the Cheekpiece to fit the new Mount, and clear the existing action was the most technical bit of routing/inletting of the entire project with several channels of various depths being required to clear the action, rail, and bolt (cycled through its cocking action) but I got there in the end with a lot of work and a good 3-4 hours in shaping this wee block of wood to fit my face perfectly guaranteeing just the right eye relief and head position for the scope.

Having completed sanding on the stock and cheek piece with to 400 grit it was time to move on to the finishing stage.

I started with a Solvent Based Exterior stain in ebony, Reccommendations had been to Brush on and then rag off in order to prevent brushmarks being left behind. After the first Coat with this method I found that the ragging off tended to draw too much of the stain away from the timber again. So a quick check of the manufacturers website did not indicate ragging off at all, so for the second coat the following Day I simply brushed on, and then nursed any runs with the brush for about 15minutes untill the product had tacked off, I kept a rag near by to dry the brush off on regularly and this method led to a nice strong stain with no runs or brush marks. After four light coats of stain I had achieved slightly heavier colour than I required, but I knew that the next steps would lighten the overall finish a little.

Once the stain was thoroughly dry, I carried out a test on a sample piece of ply with an Overcoat of Briwax Danish Oil. What I like about Danish Oil is it soaks into the project protecting the timber and hardens on drying similar to a Varnish.
 The trick here was to ensure that the solvent in the Danish Oil did not disturb the solvent based stain, after application and then rubbing down with a rag the oil proved to be harmless to the stain and as predicted the effect of the stain was lightened slightly.
 Over the next three days I applied 4 coats total of Danish Oil to the stock and cheekpiece, again standing by and removing any runs in the oil with my application brush untill the Oil had gone tacky.

After the 4 Coats of Oil I allowed a full 48 Hours of hardening time before finishing the stock with a further 3 applications of Liberon Black Bison finishing Wax. Given 10-20 minutes drying time this wax can be buffed to a polish and further showerproofs the stock against the inevitable rain that likes to wreck havoc with a good FT shoot.

And now, with the shroud re polished to a near mirror finish and the refitting done, she is 95% complete.
Still to be done will be properly engineered Linkage connectors, a custom trigger guard and Linkage Guide, and a nice fancy ButtHook assembly.

I made a custom Butt Plate/Hook from some scraps found in the Garage: a bit of 8mm Aluminium Angle cut down and a Dipole off an old Tv Aerial sleeved over some 10mm Threaded Rod.
A bit of cutting, Tapping, and filing  and then polish to a mirror finish.


Comparing the bullpup to the original configuration; the biggest improvement I have gained with relation to performance was the de shrouding of the barrel. Although not improving accuracy did improve consistancy dramatically.
 The bullpupping has certainly not caused any detriment to how the rifle shoots, but has allowed for a more secure and stable feeling during standing shots, a much more compact and adaptable hold when seated for steep uphill shots. Overall it has a more solid feeling  with no flex and shifting of plastic parts.



« Last Edit: July 20, 2013, 10:55:30 PM by Novagun »