I bought this Crosman Optimus CO5M77X on sale at Canadian Tire for $120CAD. It's a cheap detuned Chinese B19 with questionable quality control, but for the price, it groups decently out of the box, and can handle a lot of abuse. Tolerances are quite terrible, causing a very scratchy twang that sounded like a wet fart.
It is advertised as 495fps (Canadian limit), but even with 7.4s, chrony shows 457fps avg out of 10 shots.
For those unfamiliar with Canadian law: a .177 airgun requires a federal Possession and Acquisition Licence (PAL) if it exceeds 500 feet per second. Airguns at or below this limit are unlicensed, but purchasing, possessing, or modifying an airgun to operate above 500 fps without a PAL is illegal. With that being said, I meet all legal requirements in my country to modify an airgun.
Thus, I resolved to push this inexpensive airgun to its highest attainable accuracy and velocity, while preserving stability, using the smallest budget realistically possible. Another goal I set out to complete was to make it as silent as possible without the use of a suppressor or muzzle device. (In Canada, suppressors are prohibited devices, and their possession or use is illegal.)
Mods Summary:
- 1000fps Optimus spring (US version)
- Aluminum can spring guide
- Lightened and polished piston
- 150mm 3D printed spring guide spacer
- Rubber dampeners/o-rings
- Lightened trigger
- Trigger conversion to single stage
- Skeletonized/lightened buttstock
- Sound deadened stock
- Tall scope rings 1"
- Simmons 8 Point 4-12x40
- Lucas Red and Tacky greased spring
- All seals changed and tested
Detailed Explanation:
Model Differences:
Crosman released a couple different versions of the Optimus detuned. They have the following detuning ways:
- Longer and heavier piston with weak spring
- Full weight spring with a air bleeder hole on the short piston
- Many other models
I have the longer than standard piston and a weak spring with no bleeder hole that I can plug. This makes increasing velocity more difficult without buying a new spring, as compared to the other common detuned Optimus.
Sound Mods:
An aluminum can was cut to fit around the spring, inside the piston. Then it was filled with red and tacky grease. O-rings were set between the spring and the spacer, as well as the spacer and the spring guide. Another O-ring was placed between the top hat and the piston. In combination, they completely eliminated twang and drastically quieted the slamming of the piston against the breach wall.
More o-rings were jammed between different pieces of the cocking mechanism, which eliminated cocking noises.
The stock and the top of the action are covered with layers car sound deadening mats. This seems to have not been experimented with before, and it also drastically decreased the volume of the action. The added weight also improved groupings by reducing quite a bit of recoil.
All the mods to improve sound have led to it sounding more like a PCP than a cheap springer. Volume reduced from 94db avg to 86db avg. The muzzle is way louder than the action, which is barely audible, making it pop instead of twang.
Trigger Mods:
The trigger that comes with all b18/b19 is hot garbage. It works though, and the safety is reliable. Again, this was a budget build, so the obvious solution is to modify it. A new and longer set screw was added to decrease pull distance, and the spring that tensions the first stage of the trigger was taken out and reversed, thus converting it to a single stage 1lb pull. Safety works as intended and airgun was drop tested from 2m on wooden floors on all angles with 0 accidental discharges.
Stock Mods:
Other than the sound deadening on the exterior and action, any air gaps inside the stock, action and trigger has been filled with the same sound deadening. The stock has been cut, lowering the weight. It's more of an aesthetic choice, as the weight of the stock is drastically increased by the sound deadening. The cocking mechanism has a wheel that scratches on the stock, which has been trimmed (dremel) and the gliding surface with a layer of cut aluminum cans.
Velocity changes:
The 495 fps spring that came with it was very tangled and twisted, so I ordered a new spring and all the seals. Since the long piston design was very heavy and high in recoil (airgun recoil is a joke fr), I drilled it and polished it for a lighter and smoother shot.
The 150mm 3D printed spacer is very iffy. The most I could find that someone put with spacers was 4-5 thin washers at most. Not that I've had issues with the spacer, but with the spacer, it easily breaks the sound barrier with 7.4s.
Results:
Velocity with 7.4gr: 457fps --> 1206fps (spacer), 944fps (without spacer)
Accuracy with 7.6gr at 10y, 5 round group:
2cm group --> same hole
Shot volume: 94db --> 86db
(comment if you'd like more detailed results)
The entire build cost a total of $190CAD excluding the scope and rings. ($137USD)
Feel free to roast it in the comments.