The troubled gun
The Line SI Promaster was one of the early open bolt blow-back semi-auto paintball guns, having made it’s appearance on the market shortly after the Tippmann 68-Special and PMI-III (later VM-68). The initial production gun suffered from several design flaws which quickly created a bad reputation and eventually lead to the gun’s demise. That was a real shame, because when they worked, the Promaster would shoot better than any other stock gun I have ever owned. For accuracy and consistency, no off-the-shelf blow-back being produced today even comes close. My Promasters always produced extremely consistent velocities, and Line SI always made very good barrels.

In 1991 I bought one of the early Promasters because it was the first gas operated semi-auto paint gun the company that manufactured my Bushmaster pump. (Line SI had also manufactured a double-action trigger-cocking semi-auto called the Advantage. I owned one of those for a while. It had a long, hard trigger pull.) The Promaster would use Bushmaster barrels, of which I had several. The original cost was around $400.00, and at least 4 different friends of mine also bought the gun. With Line SI’s reputation, We couldn’t wait to get our hands on the Promaster.

Most people who now see the Promaster assume incorrectly that it is just another in a long line of Spyder clones. The Promaster, while having almost identical operation and similar dimensions internally, preceded the Spyder by several years. From what I can remember, the Promaster also preceded the F-1 Illustrator, which most people give credit for the development of the Spyder . Although the Spyder design and operation is the same as an F1 Illustrator,  the Spyder should probably be considered a Promaster copy. The internal valve, valve pin, main spring, bolt, and hammer on the Spyder are very close to that of the Promaster. As a mater of fact, a lot of those parts will actually function inside of a Promaster with no modification.  This is not the case for the F-1.  The initial production Spyders even had the same color scheme as the late model Promasters. The first time I saw a Spyder, from a distance I thought it was a Promaster.

Promasters had several nice features. The barrel was removed with a simple thumb screw, exactly like the Bushmaster. The snub can be removed from the body with a thumb screw, giving easy access to the bolt, and feed chamber for cleaning. The grip frame is hinged for easy access to the trigger/sear area, although getting into this area of the gun for anything other than trigger modifications is never necessary.  The trigger shoe is held on by a single screw and is the same piece that appears on later Indian Creek Designs guns.

Teething Problems
Being one of the early blow-back semi-autos, it’s easy to see how the problems in the Promaster design were unforeseen. One of these was the connection between the rear most part of the two-tube design. While the slot for the link pin between the bolt and hammer is milled completely out on modern two-tube blow-back guns, on the Promaster it was not. There was a slot machined for the link pin, but it did not extend all of the way to the back of the gun. This caused the link pin to eventually break, as it slammed back time after time against the connecting aluminum material. The solution was to remove this material, which Line SI eventually began doing on their production guns. I did mine with a hacksaw.  This modification also made pulling the bolt/hammer combo out of the gun much easier, where before the link pin first had to be removed through a hole in the top of the gun (like the F1 Illustrator).

Another immediate problem with the gun involved the double feeding of paintballs. This was caused by the poor design of the ball detent, which was a spring-loaded metal ball in the bottom of the snub. Getting two balls per trigger pull was a fairly common occurrence, especially when using smaller balls. Naturally this led to a high occurrence of ball breakage. Later model snubs addressed the problem by placing a spring loaded lever on the side of the snub. The thumb screws seen in the pictures are used to hold the Bushmaster barrels in place. These often vibrate loose unless you really tightened them down hard – which of course means you have a hard time getting them loose. Many people replaced the thumb screws with set screws for more reliability.

The early Promasters, although flawed in design, were well built. However, within several years of production, something went awry with some guns. My friends and I heard rumors (Even from Ross Alexander with Line SI) that there were other companies producing knock-offs, or somebody was producing shoddy licensed copies, and so on. This further added to the bad reputation of the gun. After owning three different Promasters, many parts have made their way into my collection that were somewhat odd. For instance, I have six different snubs, and no two are exactly alike. Some have a dip in the breech where the ball drops while others are smoothbore.. some have rounded edges on the ball detent holder, while some are squared off .. and they each seem to have slightly different internal dimensions. I have also seen this type of inconsistency with hammers, bolts, and other parts. One of the things I had to do to get the gun operating smoothly was hone out the inside of the snub. Some of my unhoned snubs will not even allow the bolt inside, while some will kink the bolt in a bind when the snub screw is tightened.

Blow-back from Blow-back
Like many guns of it’s type, the Promaster also suffered from excessive gas in the ball feed tube, or “blow-back”, as it’s commonly called. The problem can lead to a ball chop by interfering with the ability of balls to drop into the breech in cycle with the gun under rapid firing. This especially happens when the loader jams or run out of balls and there are few balls in the feed tube and the elbow.

Some guns address this problem by mounting a “power feed”, which first appeared on the blow-forward AGD Automags, and are now standard equipment on most blow-back semi-autos. Other guns tried to address the issue by adding O-rings to the bolt. On my Promaster, I have made a multi-prong attack on the problem over the years. The first thing I did was to drill holes in the feed tube and elbow in order to relieve some of the blow-back pressure. This seemed to help a little but did not completely solve the problem. Then I made an attempt to use a small knife and carve a slot in the plastic part of the bolt for an O-ring. My first attempt failed horribly, resulting in a broken bolt. The second attempt was made on a bolt which was made from a different kind of plastic, and survives to this day, many years and cases of paintballs later. However, this had only a minimal effect of eliminating blowback.

One solution that worked very well to address the problem was to add a VL2000 agitator loader to the gun. This assured no more jamming in the loader, which meant the feed tube and elbow were always full of paint until the loader ran out. This addition alone allowed the Promaster to achieve a relatively high rate of fire without chopping balls.

Another solution to the problem is a simple one I should have realized sooner: keep liquid Co2 out of the gun! Liquid Co2 expands so much when the gun is fired that it produces a lot of excess gas (and noise). Eliminating the liquid, by whatever method, greatly reduces the “blow-back” effect. Along these same lines, lowering the operating pressure is another way to combat the problem – more on that later. As a side note, some people modified Promaster snubs with powerfeeds to address the problem.

Dead Fire
Another common problem which plagued the early guns was a recurring refusal to re-cock. It did not take very much change in the weather to throw the gun completely off. Part of this problem stemmed from the fact that the Promaster was so tunable. It has a valve that could be rotated to position any of four different size holes to the top, where gas transferred to the bolt – it has a tension adjustment for the main spring, and an adapter which adjusted the valve spring – and it had valve pins with different amounts of flat sides for controlling re-cock pressure.

Problems occurred when the velocity was off because of a change in gas pressure (temperature). It could be a very frustrating experience, and I saw several of my friends sell their guns following a tournament in which they could never get the guns to properly function. I liked the way the gun shot and was determined to make it work. The Promaster was designed much like older pump guns in that it had a tiny gas feed hole going into a relatively small valve chamber. That meant the gun required high pressure to operate. The gun used a limited volume of gas to both propel the ball and re-cock the bolt/hammer. In warm weather this was not a problem. An abundance of pressure easily overcame the volume deficiency. However, in cooler temperatures, anytime you adjusted one of those functions, you noticeably affected the other. For instance: you could change to a larger valve hole for higher velocity, but then the gun would not re-cock, and when you put in a different valve pin to make it re-cock, your velocity would drop again.

Some people tried to overcome the problem by shooting liquid Co2 in cooler weather, but this lead to ball feed “blow-back” problems (as mentioned earlier). My solution was to basically make more efficient use of the available pressure. I did this by lengthening the slots on the valve pin with a dremel tool, working on the theory that not enough of the available gas was getting to the hammer for re-cock. This is probably the single biggest improvement I have ever made to the gun. It instantly eliminated my cocking failure problem, despite the weather. However, achieving a usable velocity continued to be a problem in cooler weather.

Under Pressure
For about a year, I ran one of my Promasters on my 4500 psi HPA system. The gun required nearly 1000 psi to function properly and recharge quick enough under rapid firing. It worked very well but was not extremely efficient. From a 3000 psi fill on the 68 c.i. tank I would be lucky to get 400 to 500 shots, which is much lower than the 700 to 800 I’d get from a 20 oz Co2 tank (unfortunately the field I played at could not fill beyond 3000 psi at the time).

Since a need for higher operating pressure was the main problem this gun faced, I experimented with several methods of lowering the operating pressure.  The trick to that, like on all paintball guns, is of course to improve the airflow.  Some access to improvement was available through the multi-hole valve and changing springs, but none of this proved very effective. In fact if it was that simple I would have figured it out years ago. The problem remained that no matter how good the air flow, there simply wasn’t enough air to flow without starving the system. The valve chamber needed to be larger.

One of the earlier experiments I tried for enlarging the valve chamber area involved the use of the body from a WGP Sledgehammer low pressure regulator – the type that used to come stock on Auto-cockers. Amazingly it threaded right into the front of the Promaster, and other than removing the regulator parts, only required plugging the relief holes with set screws. The seal is not as reliable as I would like and requires a combination of an O-ring and lots of pipe tape. However, it at least works good enough to demonstrate that enlarging the valve chamber is a step in the right direction. When I first tried it, I found I could get fairly usable (mid 270s) velocity using as little as 600 psi. For a Promaster, that was nearly miraculous.

Eventually, I discovered that valve chamber extensions made for Spyders would fit the Promaster (imagine that!), although some models require some large amounts of pipe tape to seal properly.  A large Shocktech volume chamber combined with the Taso high-flow valve/valve pin also made for a Spyder (which simply drops right in) offered a great improvement in the ability to move a larger volume of gas through the system.

Getting Personal
I have made a number of modifications to my Promasters over the years that were not aimed at eliminating problems, but merely at improving performance or usability. Some of the modifications were made after seeing other Promasters, so these are not necessarily original ideas, although I did them myself.

The trigger on the Promaster was probably the best on a semi-automatic of it’s time. It was far better than the F-1 Illustrator ( I have owned 2 of those). However, it could be improved with two small set screws. One screw goes into the body just above the forward part of the trigger, while the other goes into the grip frame below the forward part of the trigger. These screws can be adjusted for take-up and over-travel and give a very short, crisp action. Warning: adjusting the over-travel screw too far can cause a full-auto response. While this may sound attractive, the gun cycles way too fast to feed paint in anything except liquid form.

The rear velocity adjuster for the Promaster is inside of the ASA. This means the Co2 bottle has to be removed before adjustments can be made. I have tried several methods of re-routing the gas feed so I didn’t have to remove the bottle just to get to the adjustment. My first idea was to re-route the gas into the front of the body by drilling and tapping a hole for a 1/8 inch NPT hose fitting. This was convenient for using a bottom-line type adapter. At times I also put a 45 degree elbow in the hole with an ASA on the end and used it as a vertical bottle gun. This modification lasted for about 5 years, until May 1999 when the threads gave and hose blew straight out. I guess years of swapping fittings finally took it’s toll. I recently drilled the hole a little deeper and again tapped it – I hope is holds for a while.

The next idea for re-routing the gas input was the addition of a vertical bottle adapter.  I got this vertical ASA one from Indian Creek Designs. It requires the drilling of three holes, and the tapping of two of them. The center hole for the gas inlet must go completely into the valve chamber. Note: Although Indian Creek Designs is known for an entirely new line of guns, they did at one time carry Promaster parts.  Once another route for gas was established to the valve chamber, the rear threads on the ASA were no longer needed – so I cut them off and polished the end. The modification requires that the rear ASA air passage be blocked, which is a simple task of tapping the air passage and inserting a set screw. This gave the gun a somewhat shorter profile. On a side note, the first production Promasters had the purple finish seen on the body of this early model.

The Ultimate Promaster?
Unlike many of the guns in my possession over the years, the Promasters were not simply “loaners” or experimentation pieces. They have served as my main guns and have both been through thousands and thousands of rounds of paint, both in rec, tournament, and scenario play. Even when I owned an AGD Minimag, the Promaster was still my first back-up and often used. In warm weather, I preferred the Promaster over the Minimag because of it’s ability to be incredibly accurate without being paint picky. Not until I acquired a PPS Blazer did I actually semi-retire the Promasters… or so I thought..

At a recent 24 hour scenario game, I aquired yet another Promaster (my fourth) and began working on what may be the ultimate version of the gun. The unique thing about this example was the snub. It was one of a hand full modified to have a powerfeed. At the same time the snub was also threaded to receive WGP Autococker barrels. For the low price of $50 (which included a stock barrel), I couldn’t resist.

The gun suffered the same familiar problem when I got it, and in fact would not even re-cock the day I bought it, despite a temperature in the mid 70’s where I was playing in Florida.  A quick run over the valve pin with my battery-powered Dremel tool and changing to a softer valve spring solved the problem. I was able to use the gun that day and thoroughly enjoyed it, although overall velocity was somewhat low. When I returned home I installed the TASO valve and valve pin, and a Shock-tech valve chamber extension which put the velocity up past 350 fps, despite colder weather (it had just snowed that weekend at home in Tennessee while I was away in Florida). A spring change and a regulated Co2 source brought the velocity down to legal limits and the gun hsa since functioned flawlessly.

With the addition of a .45 grip frame, sight, foregrip and Dye Barrel, the gun was ready to rock. Personally, I just like the way it feels, which seems to be much more solid than today’s lightwieght cookie-cutter blow-backs. I also like the back-bottle set-up, which adds to the overall comfort, and gave me something useful to do with my one Co2 tank that has a built-in regulator/valve. This configuration worked well, but was a little to bright and flashy for my taste..

The latest incarnation of Promaster #4  has all of the Spyder parts, 16 oz. anti-siphon Co2 tank with CMI regulator/pin valve and an ever increasing amount of politically incorrect cosmetic work, including an actual M16/AR15 forgrip.  It also uses the powerfeed snub and an older WGP Sniper barrel which had a 1-inch OD. I turned the front end down on a lathe for the cosmetic effect. These pictures of the gun show the “carry handle” and “flat-top” versions of the the gun. I have used this gun frequently throughout 2001 and it not only continues to work great, but has never needed anything more than a slight adjustment to the CMI regulator to account for the varying limits at different fields. It’s also a very easily pointable (?) and comfortable feeling gun in this configuration, as well as being just plain fun because of the looks it receives.

Meanwhile Promaster #1, which was modified with the vertical ASA and can be seen elsewhere in this article, has been converted and re-painted for use as my gun for night time scenario play. It includes the additon of a nightvision scope, a Palmer Stabalizer regulator, a rear cocking bolt from a Scorpion (a Spyder clone) and a few other accessories. Unlike the #4, this one has the stock Promaster valve and valve pin, since attaining what would normally be considered a “usable” velocity isn’t important for night play, where limits are lower. It also has the WGP regulator body for a valve chamber extension as described earlier.

The morale of the story here? If you have a Promaster which doesn’t function, thank Kingman for building such a nice copy. Because of the success of the Spyder, many aftermarket parts are available which can turn a finicky Line SI Promaster into something that actually works – and works well.  Not only that, you can have a lot of fun as people at your local field try to guess what kind of gun it is, and try to argue that it’s just another Spyder copy.