Tuning my Air Arms TX200 HC .22
The TX200 is a lovely, accurate, rifle but is it possible to make a very good rifle even better (quieter, reduce recoil, even more consistent) by:
Making a combined 'O' ring conversion/piston extension to improve the seal and shorten the stroke
Replacing the metal piston weights and spring guides with Delrin or Oilon engineering plastics custom items
Add a PTFE Piston sleeve - Tried, insufficient clearance between spring and piston even for a .25mm thick PTFE shim
All the modifications are non destructive and reversible so the rifle can quickly and easily be reverted to the original specification if required and are designed to make the rifle smoother and more accurate NOT to increase the power. The TX200 (mine is a 2012 model) must be one of the quickest and easiest rifles in the world to take apart so is ideal for experimentation only taking a couple of minutes to break down (4 screws and one bolt - plus one circlip if you want to remove the compression chamber, but this is not necessary just to change the spring, guide and top hat).
TX200HC .22 Home made Oil filled nylon (Oilon) replacement Top Hat and Spring Guide
Not visible in the photo, the Oilon spring guide has a flange machined on the lower face which is a tight fit in the recess of the TX200 trigger housing to locate it firmly in position and reduce vibration. The Oil filled Nylon 6 (Oilon) is a hard wearing and self lubricating engineering plastic and hopefully it will have sufficient elasticity (It is softer and more elastic than Delrin) to cushion/dampen shock forces and vibration. Time will tell if it has the necessary strength and fatigue resistance required to cope with the stresses involved especially on the piston head which is also subject to sudden high temperature on firing.
TX200 HC .22 Home made Oil filled nylon (Oilon) piston extension (Shortens Stroke) and Viton O ring seal conversion
The piston extension/O ring conversion piston head is approx. 17.5mm long and compared to the original seal reduces the stroke by about 12.7mm to c. 85.3mm. The piston head is a very tight fit (piston cooled in the freezer, oilon head heated in boiling water to fit) over the existing metal seal retaining dovetail and is secured in position by three equally spaced M2 grub screws tapped into the piston extension. The anti- bear trap still works but only on the last ratchet click. The spring is still the originally fitted air arms standard spring.
The O ring conversion carries a Viton 19mmx3mm O ring set back 2mm from the piston head. The piston head itself is hard wearing and self lubricating and is a nice sliding fit in the bore even without the O ring fitted so will act in a similar manner to the existing Delrin bearings if it should contact the compression tube. The conversion, with metal top hat replaced with oilon, has resulted in a reduced overall piston weight and because of the reduced stroke has a faster lock time. In addition efficiency seems to have increased as although the stroke is shorter I have not had to increase the preload of the existing spring (in fact I believe the overall preload is fractionally less than originally due to the thinner top hat base). The rifle has been lubricated with Red Line CV2 synthetic Red Moly grease with the addition of a little Honda 60% moly paste on the outside of the spring and the ends of the compression tube. I like to think the small domed pip I have machined on the front face of the piston, which fits into the transfer port when the piston is fully extended, acts to help direct air towards the transfer port during the firing stroke and then gradually reduces the airflow and provides dynamic braking of the piston in the last few microns of it's travel as it partially enters the port to reduce piston slam and rebound. Whether it actually does anything is debateable, the theory works in my head though and it seemed a good idea at the time!
Results so far are very encouraging and have fulfilled all of my original objectives. The rifle is quieter with no twang at all and on initial assembly was running at an extremely consistent 11.2ftlbs average with JSB Exact Express 14.35gr pellets with a variation measured over 11 shots of only 4 feet per second (with unsorted straight out of the tin pellets). After running in over a couple of hundred shots the power has now increased slightly to a consistent 11.28 ftlbs with JSB Express pellets. Prior to the conversion, and with no power washers used, JSB Exact Express pellets produced the most power and were closer to the UK limit than I was comfortable with as I like a greater margin of safety. There is no spring twang and accuracy seems to have improved even further, I am assuming due to the reduced recoil/reduced lock time. If I need to replace the 'O' Ring piston seal in the future then a new top quality Viton O ring only costs a few pence and it is very quick and easy to change. In material costs only this whole tuning exercise has cost me less than £10 (ignoring my labour time!) which compares very favourably to the £200+ it cost me for the, very good, V-Mach V-Glide tuning on my HW97K. The TX200 is easier and cheaper to tune, and much easier and quicker to dismantle, than the HW97K. I haven't dismantled the rifle again to check how the new piston head is standing up to the stress as the rifle is working so well at the moment I don't want to risk altering anything. When the time comes to dismantle the rifle again, when/if performance starts to change significantly, I'll update this page with my findings.
The rifle does not appear to be pellet fussy and with JSB Exact, JSB Exact Express, JSB Exact RS, RWS Superdomes and H&N Hunter Extreme pellets at, an admittedly measly, 10 yards, the rifle produced 5 shot groups which all literally appeared as a single pellet sized hole (see picture below). This slightly improves on the already good results achieved prior to the conversion and I have never produced 5 consecutive groups with 5 different pellets of this standard before. Peak domed pellet power is obtained with the JSB Exact Express 14.35gr which also appears to be the most accurate. The standard JSB Exact 15.89gr pellet is not far behind in power or accuracy and the rifle may be fractionally quieter using these. Power falls off with the JSB Exact RS 13.43gr pellet which probably indicates it is too light for the spring piston combination and may be suffering from rebound/piston bounce on firing. The table of a chronograph (ProChrono Digital) string below shows the good consistency being achieved (pellets unweighed, unsorted, straight out of the tin with only obviously damaged pellets being rejected).
The Air Arms TX200HC is an excellent rifle straight out of the box. These modifications have made it even better and it has fast become my favourite.
Note - The air rifle was deliberately zeroed off-centre to ensure a consistent aiming point at the cross hair otherwise you shoot the aiming point out and have nothing to centre on for subsequent shots. The important thing to note is the size of each group of 5 shots. Also note how the different weight pellets hit at a slightly different height even at a range of only 10 yards due to the lighter pellets flying faster than the heavier pellets so having less time to fall under the influence of gravity before hitting the target. The slight left/right variation between group is caused because the target was standing on a slight slope so not absolutely vertical.
Example Chronograph String below showing the consistency of the TX200 HC .22 with the O-Ring Short Stroke Conversion, and oilon top hat and and spring guide after running in for a couple of hundred shots. Chronograph was the ProChrono Digital. Pellets were JSB Exact Express 5.52mm 14.35gr unweighed straight out of the tin.
|TX200 HC .22 JSB Express 14.35gr|
Showing excellent consistency with a maximum spread of only 5 feet per second with unweighed, unsorted JSB Express 14.35gr pellets.
Had the pellets been cleaned, weighed, lubed and sorted the consistency may have been even better but I am delighted with the results as they stand.
Update: Now nearly 3 years on, and at least one thousand more shots with no sign of performance drop off (still using original O ring), my modified TX200HC continues to impress and it is without doubt my nicest to shoot spring powered air rifle which replicates the single pellet size 5 shot groups time after time.
I also have an AA Prosport .177 rifle that straight out of the box, with no more than a clean, relube and polish to smooth off any rough edges, is as accurate as my full V-Glide (not just V-Mach) HW97K. So far I have resisted tuning it as it is already so good but I keep wondering if applying the same tuning could make it even better especially as the recoil of a .177 tends to be a little harsher than a .22 at the same power levels. It would be hard to make it much more accurate than it already is (it is already that good) but maybe it could make the firing cycle quieter and reduce recoil which may also reduce hold sensitivity. If I do decide to tune it I will update these pages with the result.