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 VOILĂ – 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.
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 installedThe 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.
In the first part we checked out the rifle, removed the busted scope and wonky sights, fitted a scope and checked for accuracy, albeit at short distance only. In this part we’re going to disassemble the rifle and check out the internal parts.
Crosman Optimus with Leapers 6-24X50AO scope mountedInitial disassembly of the Optimus is pretty straightforward. Remove three Philips’ head screws, one at the rear of the trigger guard and one on either side of the forearm.Now the action lifts easily out of the stock.Note the Anti-Bear Trap bar that links the cocking mechanism to the trigger group.Next on the agenda is to disassemble the barrel from the compression tube. Note the large bolt at the joint, this has to come out.
First, “break” the barrel open, this releases the tension on the bolt and allows it to turn relatively easily.
Use a well fitting screw driver to turn the bolt out. See how the threaded portion is smaller than the bearing portion? It only goes in and comes out in one direction.This example gave me fits at first. It was necessary to close and open the rifle before the breech would start to slide out of position and allow it to be further withdrawn.There’s a plastic/nylon/synthetic of some kind washer on either side of the breech block. This acts as a bearing and helps to keep the ears of the compression tube from scoring up the breech block. It fits in the recess around the hinge pin. Note the amount of grease in the groove and the total lack of any around the bearing.After separating the barrel from the breech block the two pieces didn’t completely separate yet as the Anti-Bear Trap was still attached and I didn’t want to force the cocking piece out of its place in the compression tube.Next, remove the stock lug/rear spring guide bolt. Note the presence of a brass washer between the trigger mechanism and the compression tube.I like to dismantle the rest of the rifle while in a spring compressor for safety’s sake. You never know how much preload will be on the spring and it’s easier to control this way.Put just a bit of pressure on the back of the spring guide assembly then carefully drive (or push) the pin out. Then back off the compressor allowing the spring to decompress. There was 1.370″ of preload on the spring.Once the tension was off the spring I was able to wiggle the trigger pack out and then unhook the Anti-Bear Trap mechanism from the cocking arm. Note that piece that’s sticking up at about a 45* angle. Pay attention to how it lays in place as you’ll have to position it just right to reassemble the rifle.Note that “T”stud, that’s the bit that guides and retains the ABT bar. Above it is the “cocking shoe” which is actually just a part of the cocking arm itself.Here’s a look at the mainspring still “in situ”. Note the poorly finished coil giving just a bit more than 50% contact around its circumference.Here’s a look at the spring on the rear guide, pushing against the trigger seat.Here’s a look at the tophat, spring, rear guide and piston.And here they are fitted together as they are inside the rifle, but uncompressed.The first closeup of the seal makes it look like there’s some kind of metallic film covering it. That’s just light reflecting from the flash.Angling the seal slightly you can see that it’s just discolored from combustion and that it’s got a flat spot on the edge that’s at about 8 O’clock in this picture.
So, here’s some data: Piston Clear depth – 5.185″- 131.7 mm 1.370″ – 34.8 mm – Preload 36 coils – 1.23″ – 3.124mm wire Piston with seal – 193 grams Piston with seal and tophat – 226 grams Tophat – 33 grams of ill fitting steel
Overall the rifle’s in fairly good shape. I’m guessing that the seal is partially to blame for the high SD in the chronograph string we looked at. Now we move on to tidying things up and getting the rifle ready for reassembly.
There’s a shooting sport that I find quite interesting as it limits to a certain extent the typical technology race, forcing the shooter to concentrate on improving their technique and abilities. It’s called “Hunter Field Target” and the World Hunter Field Target Association (WHFTA) is the organization that is responsible for the rules and such. I found it particularly interesting as it gives me the incentive to use a lighter rifle and simpler scope in the pursuit of steel silhouettes to slaughter.
And so I decided to search for an appropriate low budget rifle to dedicate to the pursuit of this sport. The rifle I settled on is the Crosman Optimus. I’d had previous experience with this model as I purchased the last one in .177/4.5mm on Mercado Libre back in 2020 as a project to keep my mind busy during the national lock down that year. After tweaking and tuning and polishing it up it was “sold down the river” to recover the cost of purchase. The new owner was quite pleased with it due to the ease of cocking and decent accuracy it gives.
Recently a used rifle showed up on Mercado Libre for a very reasonable amount (for Coffee Country) with a Tasco 2-6X32AO scope and a hard case. As the idea is to reduce power and clean it up to shoot below 12 FPE in keeping with the WHFTA rules the most important thing was that it be in good shape externally as we’ll be working on the seals, springs and other internals. A bonus was that the rifle was listed as local to us and it turned out that the original owner is an acquaintance who purchased the rifle back in 2011 and hasn’t used it much since.
Crosman Optimus with Tasco 2-6X32AO scope as receivedThe rifle appeared to be in overall great shape, some light surface rust was quickly resolved with some oil and steel wool
Shooting at 5 meters yielded an interesting group, but also revealed the fact that the scope was junk. So I pulled the scope and opted for the “irons” (actually – plastics), but the Crosman sights leave a LOT to be desired, to say the least.
Sight cranked almost completely to the left to get it close to center on the paper
Well, no big deal. The original plan was to pull the sights off anyway as the shooting will all be with a scope mounted and I prefer the no snag, slick barrel look on a rifle so equipped. And with the sights being so lousy there’s not much reason or keeping them on as there are other options in the cabinet should I become desirous of shooting with open sights.
So I grabbed a piece of copper bar from the tool chest and a hammer as well. A couple quick taps of the hammer and the front sight was removed towards the front.
Copper bar used to tap off front sight
Removing the front sight revealed a bit more of the surface rust that developed from our humid climate and lack of good rust preventative from the factory. This cleaned up quickly with a bit of my anti-oxidant concoction and a bit of steel wool.
Next up was the matter of removing the rear sight. Not a problem at all. Just completely unscrew the rear vertical adjustment knob then carefully raise the rear sight leaf being careful to not lose the spring inside. Now you have access to the two Philips’ head screws that hold the sight body to the breech block. Remove those, remove the sight from the rifle, place the screws back in their holes and the spring in its place then secure the lot together by inserting the vertical adjustment knob and screwing it down. Now you can put them all in in a ziplock baggie “for future reference and/or disposition”.
Front and rear sights ready to be bagged up and stored
So, once we determined that the scople/sights needed replaced, we dug out an old Leapers 6-24X50AO scope that’s been begging to be put back in the game. Some might refer to it as a “Hubble”, but it’s been fairly reliable and fit the Optimus OK, leaving just enough room to cock the rifle and load a pellet.
Optimus with Leapers 6-24X50AO scope mountedThe breech barely clears the front of the scope with the front scope cover removed.The first group revealed promise, and that the Tasco scope had issues as well.Hatsan Vortex Strike 9.25gr pellets at 10 meters showed a bit of promise, but nothing stellar, unless you count looking like a galaxy “stellar”.Gamo Hunter 7.53 gr pellets weren’t exactly all that great either.Going back to the Vortex Strike showed promise
I shot down the hall at about 10 meters and after letting the barrel “settle in” with the new pellets things got interesting. I noticed that a difference in feel when loading the pellet often resulted in the group opening up. This is a pellet issue and since I had multiple targets on this sheet I started shooting loose fitting pellets into a former group. So the bottom left group in the picture above is from 5 pellets that fit tightly into the breech on loading. The one diagonally to the right of it has about 7 pellets, the original 5 I shot there and then two loose ones from when I shot the lower left target.
Next step is to check on the general “health” of the rifle via a quick string shot over the chronograph. This is a rifle that was purchased new in 2011, shot until all the assembly oil dieseled off and then stored for ??? until the former owner decided to sell it. So here’s the results on the chronograph.
Crosman Optimus No 2
First shots
Altitude
Distance
2022-10-26
JSB Exact 8.44
4135.0
0.50
Shot #
FPS
FT-LBS
PF
1
778
11.35
6.57
2
806
12.18
6.80
3
818
12.54
6.90
4
804
12.12
6.79
5
802
12.06
6.77
6
793
11.79
6.69
7
794
11.82
6.70
8
822
12.66
6.94
9
783
11.49
6.61
10
799
11.97
6.74
Min Vel
Max Vel
Spread
778
822
44
AVG Vel
TrueMV
StdDev
799.90
799.95
13.84
The “True Muzzle Velocity” (adjusted for average and distance from the first sensor) is barely in the WHFTA range. But there were a number of shots over the limit which would put this rifle squarely into the “Open Category” as recognized here in Coffee Country. Note that this is using a typical 8.44 FT type pellet. The factory specs on this rifle claim “1,000 fps” – but never state with which pellet. So far I’ve never found a rifle that really reached the velocity the factory claimed, most being WAY below that speed. BUT, here we’re not about speed. We’re about accuracy and that’s what we’ll be shooting for. (heh, heh, heh). The goal is to get this rifle shooting around 11-11.5 FPE which translates to an 8.4 grain pellet between roughly 768 and 785 fps. We’ll be opening up the rifle to look inside and then we’ll start tweaking it to see if we can reach our goal.
