Optimus – Paul's Airguns Blog

Part 1
Part 2
Part 3

We see that the piston has some issues, but nothing too major. One of (if not THE) most important aspects of getting an air rifle to shoot well is the barrel. So it’s time to start fettling that a bit.

The crown looks a bit ambiguous. Not clearly defined at all. So, run a cotton swab up to keep debris from falling into the barrel and off we go.

It’s not easy getting a good picture, but you can see that the crown isn’t clear cut at all. You want to see the lands clearly ending and all even so that they release the skirt of the pellet simultaneously upon exit. We took a “dremel” type abrasive tip, chucked it up in a cordless drill, added some generic grinding compound for lubrication and worked it around and around and around in the muzzle. Unfortunately the picture we took didn’t come out, but you can find videos and such of the process on the ‘tube.

The breech seal was a bit flat, so we took it out, wound a bit of dental floss around the breech stub, reinstalled the seal and now we’ve got a good airtight seal. We also took a bit of fine emery paper, wrapped it around the tip of a pencil and gently deburred the pellet chamber entrance.

So that’s it for preliminary barrel work. We’ll probably come back and do a bit more on the crown as it’s not quite as clearly defined as we’d like to see. But in the meantime we’ll move on to a couple more mods, assemble and see how we’re doing.

First up is the installation of a GRT-III trigger blade from “Charlie Da Tuna”. The old blade and trigger return spring are left over parts, shown at the right side of the newly installed GRT-III trigger.

The 2011 Crosman used a solid, probably cast, trigger blade. It was much better than the first trigger of this type we replaced many years ago. That one was stamped steel, but can’t seem to locate it for a comparison picture. It was in a Gamo CFX built prior to the switch to the newer style trigger group with the X type spring guide.

Here you see an arrow pointing at a problem we had initially with this installation. The two diameter pin came up too high to allow the ABT to work properly. You can see how the tab on the ABT runs into the top of the pin. A bit of work with a diamond grit hone worked the tab down and smoothed it up so now it slides over easily. You can also see how the original “finish” on that tab had a bur that scratched the trigger housing where it dragged back and forth as the rifle was cocked. That burr is now gone.

Fitting a Diana/Gamo/B19/Crosman type seal can be a pain. I’ve fixed up a little jig to help. A piece of 1/2″ PVC hot water pipe beveled at one end. You use a little bit of silicone oil to lube it and slide the seal on it, making sure it’s the right way up. Then slide another piece of 1/2″ PVC pipe, cold water type, over the outside of the first piece of pipe. Line it up with the dovetail on the piston, give it a quick rap with a rubber mallet and VOILÁ – pop that seal right into place.

As you can see, the edge of this Gamo replacement seal from Air Rifle Headquarters is quite proud of the piston body. So we worked it down with a piece of emory paper until it entered the compression tube snugly. A bit TOO snugly, but with some lubrication in appropriate places it went into place.

Remember how the rifle was just a bit over powered for WHFTA regulations? Well, the spring was also not properly finished, so we cut off two coils and then flattened up the end properly. Someone forgot to take a picture of the finished spring end. That should read “installed spring end” as it’s not exactly finished yet, we wanted to see how the mods made so far were working. So here’s a look at the trimmed coils.

You can see that at the factory they at least closed the last coil (on the right) and slightly flattened it. You can also see that a fair percentage of the spring was NOT in contact with the spring seat because the end was not ground down flush like it should be.

So we heated the end of the spring nice and red then dropped the top hat into it and pushed it down nice and square. Once it cooled off we fired up the belt sander and ground both ends of the spring down to give a nice, flat surface for the spring to push against the spring seat properly. As mentioned above, no pics taken of this yet as we will be disassembling and finishing up the job later on. First for some tests.

We installed the original top hat in the spring, dabbed moly paste in appropriate places and reassembled the rifle. Then we ran a quick chronograph test to see how it’s doing. The rifle is obviously not completely ready or finished yet, but the shot cycle is much nicer at this point, even though the spring guide is too loose for this spring, as is the top hat, and the seal is still oversized and needs sizing down a bit more.

Crosman Optimus	ARH seal, cut two coils off spring	Distance to Chronograp	Weight	Altitude
2022-11-12	Seal still tight	0.50	8.44	4135.0

Shot #	FPS	FT-LBS	PF

1	717	9.64	6.05
2	716	9.61	6.04
3	717	9.64	6.05
4	711	9.48	6.00
5	691	8.95	5.83
6	715	9.58	6.03
7	713	9.53	6.02
8	702	9.24	5.92
9	716	9.61	6.04
10	723	9.80	6.10
11	714	9.56	6.03
12	726	9.88	6.13
13	719	9.69	6.07
14	735	10.13	6.20

Min Vel	Max Vel	AVG Vel	TrueMV
691	735	715.36	715.40

	Spread	StdDev
	44	10.29

The chart above shows the first 14 shots after a couple of “function test” shots. Standard deviation is still high and power is still quite a bit below where we’d like to be. So we’ll shoot it a while when taking a break from our real work and then we’ll chronograph again before breaking it down for further fettling.

Part 1
Part 2
Part 4

In the first two parts we checked out the rifle for relative “health”. It’s a fairly sound rifle but there’s definitely room for improvement. I tend to look at these rifles as “assembled kits” that need taken down, cleaned up and reassembled before ever shooting them. This one’s 11 years old so we shot it anyway to see how things were. The one I played with back in 2020 was an interesting case in why I believe they need taken apart and cleaned up prior to shooting. The second shot was in the neighborhood of 1485 FPS. I’ve no idea how fast the first shot was because I’d merely cocked the rifle, inserted a pellet and shot it into the pellet trap. The sound of that first shot taught me a few things.

1. There’s a lot of oil in front of the seal on some of these “bargain” rifles.
2. It’s important to warn one’s wife PRIOR to firing a test shot indoors.
3. The dogs aren’t amused by sudden loud noises either.
4. It’s probably best to clean up a new rifle BEFORE shooting it.

Anyway, here we go!

The tophat weighs a healthy 33 grams of ill fitting steel.

The piston weighs 193 grams with seal installed

The two together give 226 grams of heavy pellet pushing weight. We’re aiming to use lighter pellets so that tophat will have to go eventually.

The piston shows some odd scuff marks I’ve never seen before. On both sides of the piston body and at the front of the piston right behind the seal.

The scuff shown on the next picture, at the bottom, rear of the piston, is fairly typical as that’s where the cocking arm causes the piston to push against the inside of the compression tube as the rifle is cocked.

Next we look at the seal. On the left is the seal from the rifle. It shows 11 years of use, abuse and neglect. And it’s got an obvious flat spot on the edge. Next to it is a brand spanking new Crosman OEM seal I picked up recently.

Since this is to be a personal rifle and is not being fixed up for someone else but hopefully will give many years of good service, we’ve got options. The three seals on the right are ARH (Jim Maccari) seals. The red one is from a Gamo kit I bought from him many years ago. The blue and green are either AirArms TX200/ProSport seals or Diana 34 seals.

“Just because” we’ll be using the Gamo seal in this build. It’s a tight fit and needs some sizing, but is the same quality of seal as Maccari is known for in all his products.

air powered projectile propulsion and platforms pertaining thereto

Category: Optimus

Crosman Optimus – Part 4

Crosman Optimus – Part 3