Today reader Vana2 whose name is Hank, shows us how he modified the Beeman P17 pistol to make it easier to pump.
If you’d like to write a guest post for this blog, please email me at email@example.com.
Beeman P17 Improvements
by Hank Vana2
This report covers:
- Complaint 1 – Heavy closing force
- My solution
- Piston modification
- Test results
- Complaint 2 – Awkward loading
I was taking a break from clearing brush when I spotted him hiding amongst the leaves. “Bet you can’t hit me” the acorn said. “Bet I can” I replied and sent him flying with a well-placed shot.
I love plinking. If shooting tins and spinners on the range is fun, roving and shooting random targets of opportunity while on a walk-about is even more so.
When out working on the property I always bring an airgun. I prefer rifles but often it’s more convenient to carry a pistol which brings me to the subject of my guest blog: the Beeman P17.
The Beeman P17 is an all-around nice little airgun that is excellent value for the money. BUT, there are two common complaints against this .177 caliber Single Stroke Pneumatic (SSP) – It’s heavy to close (charge) and awkward to load. In this blog I’d like to share the improvements that I’ve done to my P17 to address those complaints.
Complaint 1 — Heavy closing force
For its compact size the P17 puts out an impressive amount of power – around 2.7 Foot-Pounds of Energy (FPE) at the muzzle. Unfortunately, with the short cocking lever (the barrel), it takes about 28 pounds of force to compress enough air to drive a 7.4 grain Crosman Premier to 410 Feet Per Second (FPS).
Because it take a lot of effort to close the action on the P17, people hold the pistol close to their body to gain the best mechanical advantage and often trap clothing (or skin) in the mechanism.
The other concern is that the effort of loading takes the fun out of plinking and after shooting for a bit, muscles become fatigued and it’s difficult to shoot accurately.
Considering that I use my P17 for casual plinking that doesn’t require a lot of power I thought I’d trade some of the velocity for a lower closing effort.
Being an SSP, the way to reduce the power is to reduce the compression in the cylinder. That can be done by reducing the amount of air being compressed or by increasing the volume of the compression chamber so that the air isn’t compressed as much.
The length of the compression stroke on an SSP is determined by the location of the air intake vent on the cylinder. On the P17 it’s located on the side of the cylinder, about a half inch down from the top. This hole is the one that you should check for O-ring destroying burrs before using your pistol!
It would be easy enough to drill another vent hole (further down on the cylinder) to reduce the volume of air to be compressed, lowering the closing force required. I don’t like this approach as it would be difficult to plug the hole if I wanted to return the pistol to full power.
I chose to increase the volume of the compression chamber and reduce the closing force by drilling a pocket hole into the face of the aluminum piston. My thinking was that I could systematically drill the hole deeper or larger until I found a closing force I was comfortable with and if I wanted to return my P17 to factory specs it would be easy to plug the hole with a set-screw.
The difficulty loading a pellet can be fixed with a bit of polishing.
The testing jig
For myself, I could have just systematically drilled the piston and checked the force until I found what I liked, velocity being whatever it was going to be. For the guest blog I needed to measure the amount of force needed for a specific pocket hole volume and the resultant velocity. So I had to make a jig.
To make measurements, I bolted my scale to the workbench and attached a (sliding) 2×4 that had block to hold the pistol steady while I applied the force needed to close the action. The pointer on the scale moved a magnet down the vernier to record the peak weight. The pistol was then shot over a chronograph and the velocity recorded.
The piston on the P17 is held with a pin which is locked in place with a set-screw. Loosen the set-screw with a 2mm Allen Key and push the pin out. The cylinder can then be angled so that the piston can be pulled (straight!) out.
There are two things to note here…
First – check the inside of the cylinder to see if there is a burr on the air inlet vent hole that could damage the piston O-ring. The presence of a burr used to be a common manufacturing problem; it seems they have addressed that issue as this P17 was fine. If there is a burr, remove the sharpness with 600 grit sandpaper.
Second – clean the factory grease off of the piston and out of the cylinder – carefully (wood or plastic implements and clean paper towel) to not to scratch the inside of the cylinder. My P17 had an excessive amount of cheap, brown, general purpose grease instead of a thin, light coating of the white lithium grease recommended for SSPs. Some of the excess factory grease worked its way into the valve system and caused the valve to stay open until I flushed it out.
Here’s where we get into the piston details. The modification itself is simple – I put some tape on the drill bit to mark the 1/4 inch depth I wanted and drilled a pocket hole into the aluminum piston.
The interesting thing to note is the air-flow pattern in the grease on the piston face shown on the left in the picture above. The compression chamber transfer port is adjacent to the middle of the cleared area.
The piston is soft aluminum (6061-T6) that is easily marked. Make sure that you support it by the shank and protect the piston itself from damage during the drilling operation. Clean off any chips of metal and lubricate the O-ring and piston with a light coat of white lithium grease before reassembling.
Ideally, for the cleanest air flow, the pocket hole should be in the face of the piston, directly opposite the transfer port. I don’t think it’s critical where you drill the hole as long as you are not too close to the edge of the piston where you can break into the O-ring groove.
I located the hole vertically (in relation to the pin – red dotted line – that secures and orients the piston) about 1/4 inch above the piston center. Just be sure that the distance from the O-ring groove is suitable for the drill diameter you intend to use.
I baselined my P17; as received from the factory the pistol took 28 pounds to close and shot at an average of 408.8 FPS with an Extreme Spread (ES) of 10.9 FPS and a Standard Deviation (SD) of 3.0 over 10 shots with 7.4 grain pellets.
I tested 1/8, 5/32 and 3/16 inch diameter pocket (blind) holes drilled 1/4 inch deep into the face of the piston and recorded the closing effort and resulting velocities.
The closing effort decreased as expected: for a 1/8×1/4 inch pocket – it was 26 pounds effort; 5/32×1/4 pocket – 22 pounds and 3/16×1/4 pocket – 18 pounds. I found 18 pounds closing effort comfortable so I didn’t test larger pockets.
Surprisingly, the velocity didn’t change appreciably. Over the whole test (150 samples) the average velocity with 7.4 grain pellets was 408.2 FPS.
I surmise that the volume and pressure of the charge in the P17 exceeds what is required to push the pellet out of the relatively short (about 6 1/2 inches long) barrel and much of the air and the effort to compress it is wasted. Tighter fitting and/or heavier pellets will likely show different results. I’m pleased with the way things turned out!
Complaint 2 – Awkward loading
The second complaint that I’ve heard is that the P17 is that it is awkward to load.
Fat fingers aside, I’ve noticed that the real reason that it’s difficult to load a pellet into the breech is that the sharp edge of the chamfer is catching on the soft lead pellet.
The breech on my first P17 was machined flat to seal against the O-ring and didn’t have a chamfer at all. You had to carefully align the pellet to get it to load – domed and pointed pellets were difficult and forget about wad-cutters! Needless to say, that was fixed in a hurry!
The breech of my new P17 has a small chamfer the factory put in but the edge of the chamfer where it meets the rifling and the edge at the sealing surface of the barrel had a slight burr on them that would snag the head of the pellet and make it difficult to seat. It’s better than it was on the first pistol but can easily be improved.
It takes a minute to undo the two screws and pop out the barrel. Then using the fine-grit stones (shown in the picture above and that are sold for use with a Dremmel tool) polish the edges of the chamfer lightly – by hand – to take the burr off and leave them smooth and shiny. (Do NOT touch the crown end of the barrel!) Clean and reinstall the barrel and even wad-cutters should load easily.
I like my Beeman P17 SSP pistol. It’s got decent power, accuracy is excellent and it’s just a fun pistol to have around. These two little improvements to make it easier to load just add to the enjoyment of plinking with one.