ASI Sniper (Gamo Expo) .177 Project

The 'O' ring piston seal conversion we made for my old ASI Sniper (re-badged Gamo Expo) .177 to replace the original leather seal - Only the second item my son and I have ever made on a metal lathe (Myford ML7) but it came out really well.

Gave an immediate performance increase but also made the rifle far more consistent with only a 7fps variance over a 10 shot string.

The size of the 'O' ring groove is critical to get just the right amount of "squash" so that the 'O' ring seals but doesn't create excessive friction

Rifle compression tube diameter nominal 25mm

Aluminium stock 25.4mm diameter

Aluminium Piston Head machined down to 24.7mm diameter

Piston 'O' ring groove diameter 19.6mm

Piston 'O' ring groove width 3.24mm diameter

'O' ring 19mm I.D. x 3mm Cross section

Next job on the list is to replace the extremely tired original spring with a No. 7 Titan XS spring and add a top hat and ptfe piston sleeve.

Leather-Seal-To-O Ring

MKI - Aluminium O Ring Conversion


ASI-Sniper-Oilon O-Ring-Conversion

MKII Oilon oil filled nylon O ring conversion

ASI Sniper improvements so far:

1. Original condition as received leather seal, tired spring, unknown last lube

2. Stripped, cleaned and relubed - Leather seal oil soaked (seal actually in remarkably good condition)

3. Added Power Washers/spacer to aid tired spring

4. Changed Leather Seal to MKI Aluminium 'O' Ring Conversion - This also increased the stroke a couple of mm as the 'O' ring is more compact than the original seal. Some evidence of scoring on the soft aluminium.

5. Fitted new Spring

6. Fitted MKII Oilon Oil filled nylon O ring conversion - reduced weight, self lubricating bearing (should eliminate scoring and the friction resulting), added yet another couple millimetres to the stroke which may help increase power.

7. Made and fitted small Oilon top hat to support lower spring and allow rotation (pending)

8. Can anything be done to improve the trigger (pending)?

The main limitation on the performance of this rifle is the trigger.  It is a very simple direct single stage trigger which has a lot of pressure on it from the spring/cocking mechanism so it is impossible to get a really light feeling release.  The adjustment screw basically just moves the trigger closer to the release point, in fact right to the point when it will rather dangerously actually go off when closing the barrel after cocking the rifle if you over do it.  I also have suspicions that the bore of the compression tube may be slightly ovaloid rather than perfectly circular which may affect efficiency (the original leather seal may even have an advantage here as it deforms to fill any shape).  Having said that at 10 metres it does actually produce a pretty good group although not in the one hole class of my Air Arms TX200 HC.  Unless I can find a way of modifying/replacing the trigger unit I don't feel inclined to spend any more time trying to tune it as it is probably not far from as good as it will get now and much better than before I started.  Currently on loan to a friend so I can't take final chrono readings.

Condition High ft/lb Low ft/lb Avg ft/lb Spread fps
1 Original as received 3.19 2.22 2.7 69
2 Cleaned/Deburred/Relubed 4.20 4.04 4.13 21
3 Added Power Washers 4.69 4.37 4.51 18
4 'O' Ring Conversion 5.13 5.01 5.04 7
5 'O' Ring + New Spring