QB78 / XS78 / XS79 Adjustable Butt Pad

A 4-position butt plate & pad for the SMK / Beeman XS series of CO2-powered air rifles. The original stock is monte-carlo style and rather small (almost junior size) with a short pull, and so less than optimum for taller people. Adding a simple adjustable spacer that uses a couple of cap head screws to allow the butt pad to be kept in line (best for prone shooting) or dropped down to suit supported or off-hand shooting, whilst adding a little put to the length of pull, turns an OK stock into quite a decent one. Still could use an adjustable cheek piece and a hamster but this was an OK start. There are comemrcial ones around but they are expensive and out of stock everywhere at the time I needed one, so I fabricobbled my own. I dont need more than a couple of positions so rather than using sliders and clamps, or fancy clamping solutions, and as I wanted a bit more length of pull, I went with the KISS principle and just used bolts with a choice of hole positions.

I made the parts in Fusion360 but my skills are not amazing - I shaped the plate by tracing my rifles stock end, photgraphing it then importing that as an image. I then added spline curves to match the drawing and scaled to match the stock as closely as possible. So this is, afaik, only going to be any use for the base XS and QB stocks ie no other brand nor the TH or Match stocks. Anyone finds it does actually match a different brand/model too, do please add that int eh comments and I can add a suitable tag to make it easier for others to find.

The polymer probably doesn't matter - mine is eSun PLA+ but maybe HG100 or some other more heat-resistant copolymer would better survive transport in a bag in a car boot in the summer. I used 40% infill though again this isn't really that important. However 3 walls and 4 top & bottom layers is a good idea - the holes on the left side of the butt plate are just a fraction smaller to allow M5 machine screws to make their own thread - avoid using a tap as that reduces the friction and could allow them to work loose over time. Laying them flat minimises support and print time alike, and I use tree supports for the bolt-holes. You can use normal supports if you prefer though - I made all the holes open-ended to allow stubborn support material to be drilled or pushed out. Just be aware that the holes on the left side of the slide channel are a bit over 4.5mm so the M5 bolts can self-tap; dont use a 5mm bit on those!

My printer is a custom build bedslinger and is capable of very good accuracy, so the models tolerances are quite tight. If your printer is not so hot on accuracy, I would suggest slightly rescaling the sliding part width in the slicer to widen it without scaling the bolt holes, then sanding the sides of the slider part down to get an accurate snug fit in the channel.

To fit, you'll need 2x M5 12-16mm and 2x M5 35mm screws, preferably socket-head, otherwise pan-head. Optionally, replace one of the 35mm screws with a 30mm. A sharp knife and stout flat lever (I used a screwdriver) may also be needed. Optinally: a small amount of wood stain matching the stock colour as closely as possible.

1. Take out the long screws holding the butt pad on to the stock and remove the pad. Mine felt like it was glued on but it wasnt, just had been put on still tacky from moulding and adhered a bit, so I used a thin knife blade to start a gap then a flat screwdriver to lever the pad off of the wood.

2. Optional - the stock was stained after the pad was fitted so the wood beneath is untreated. For maximum longevity, put a couple of coats of woodstain on the exposed bare wood and allow to fully dry.

3. Use the wood screws to attach the plate to the stock - tighten carefully as the wood is quite soft and would easily be damaged if too much force was used. You could put a smear of silicone caulk over the butt but I found just using the screws has been sufficient so far.

4. The short M5 are used to attach the original rubber pad to the adjustable shoulder pad section. Use cap-head for preference to make installation & removal easier - anything beween 12 and 16mm should be OK. Philips pan-head would work but are a lot easier to damage whilst the screw cuts its thread into the plastic.

5. Fit the 2 parts together then use the longer M5 bolts to locate the butt at the desired height. They go in from the right side and should push easily through the holes on the right and those in the slider, then cut their own thread in the smaller holes on the left side. If you have one, use a 30mm for the lower one, but I used 35mm for both and havent had the expoed threads of the bottom one catch on anything yet. I might file it down a bit anyway, just to remove the sharp ends of the threads, as it might possibly cut up the inside of the slip left as it is.