A Look at the Colt Mk. IV Series 70


Colt's Custom Shop Mk. IV Series 70 in stainless

Colt’s Custom Shop Mk. IV Series 70 in stainless

This week we’re taking a break from travel and photography and concentrating on another of my varied areas of interest — firearms. We’ll begin with a rather rare bird, a new Colt Mk. IV Series 70. The example you see here is in stainless steel.

But first a history lesson on the Colt “Government Model” M1911 Series 70 versus the Series 80, and the controversy surrounding the latter.

Colt's Custom Shop Mk. IV Series 70 in stainless

Colt’s Custom Shop Mk. IV Series 70 in stainless

The Mk. IV Series 70 dates from the year 1970. It differed from the previous M1911A1 (1924) in that the pistol had installed a split barrel bushing (sometimes referred to as a “collet bushing”) to increase accuracy of an already very accurate handgun. The following photos from Hooper Gun Works illustrate the difference between the two bushings. The first show the differing bushings installed onto a Colt barrel, and the second is a close-up of the actual bushings:

Traditional bushing (left); Series 70 split barrel

Traditional bushing (left); Series 70 split barrel “collet” bushing (right); both installed on barrels

 

Traditional bushing left; split barrel bushing right

Traditional bushing left; split barrel bushing right

To be brutally frank about it, the split barrel “collet” bushing was a bit of a disaster. The fingers had a tendency to break off and jam the pistol. So, by 1988 the split barrel bushing gave way to a return to the solid bushing in later iterations of the Colt M1911 line.

Colt Mk IV roll mark

Colt Mk IV roll mark

Colt ended the Mk. IV Series 70 in 1983 with the introduction of the Series 80. This M1911 initially retained the split barrel bushing, but it incorporated a new feature hated by M1911 purists. It is theoretically possible that if a cocked-and-locked M1911 is dropped onto a hard surface in such a way that the muzzle strikes first, the free-floating firing pin may have enough inertia to overcome the tension of the firing pin spring and thus impact the bullet primer. This could produce an unintended discharge.

M1991A1 roll mark

M1991A1 roll mark

Colt addressed this concern in 1983 by redesigning the firing system to incorporate a firing pin block. The firing pin block is disengaged by the trigger mechanism as the trigger is pulled. So, why the controversy among 1911 purists? After all, isn’t increased firearm safety a good thing?

Apparently, not if it adversely impacts a great single-action trigger . . . or even if it’s merely imagined to do so.

Mk IV reverse side

Mk IV reverse side

Many claim that this trigger-deactivated firing pin block increased trigger weight and friction. I’ll be testing this claim on Wednesday with a video of a test I performed that pits the Mk. IV Series 70 pictured here up against a Colt M1991A1 with the so-called Series 80 trigger. Not in the video but outlined in the text on Wednesday are additional comparisons I performed with a stainless Colt M1991A1 chambered in .38 Super and another .45 ACP Colt M1991A1 identical to the one used in the video. I believe you’ll find the results most enlightening, and I’m sure very controversial among M1911 purists.

M1991A1 reverse side

M1991A1 reverse side

By now you’re asking, how the heck did Series 70 come to mean a trigger that dates from 1911 to 1983, while Series 80 describes the trigger system with a firing pin block incorporated in 1983? Good question, since the Series 70 was so designated not because of the trigger system, but rather because of the split barrel “collet” bushing — a bushing that continued on in the Series 80 pistol for some five years. The answer is convenience. It’s just easier to differentiate between the original internal trigger design of the M1911 to the later firing pin block design by referring to the two designs as “Series 70” and “Series 80” firing systems.

Here is another photo from Hooper Gun Works that illustrates one of the modifications made, in this case to the hammer:

Notched Series 70 hammer on left; shelved Series 80 hammer on right

Notched Series 70 hammer on left; shelved Series 80 hammer on right

(Note: There is a competing firing pin block design out there for the M1911 in which the firing pin block is deactivated by the grip safety rather than the trigger. It’s called the Swartz Firing Pin Safety, and it’s used by some competing M1911 producers such as Kimber and Smith & Wesson. Ironically, Colt pioneered the Swartz system in 1938, but dropped it after only three years because the U.S. military balked at the added expense, as did the civilian market. Modern manufacturing processes have allowed the economical reintroduction of the Swartz system, but Colt have stuck with the Series 80 firing system.)

Around the turn of the century it began to dawn on Colt that M1911 purists were leaving the fold for competing M1911s produced without the “trigger degrading” Series 80 firing pin block. This resulted in Colt reintroducing a “Mk. IV Series 70” that really isn’t much like the one produced from 1970 to 1983. For one thing, there was no return to the split barrel bushing. The Series 80 slide and frame were also retained, even if the firing pin block was removed. Of interest to some may be the fact that Colt continues to use in the reintroduced Series 70 the internals of the Series 80. That means the shelved hammer pictured above is still used rather than the earlier notched hammer. What Colt is giving you with the new Mk. IV Series 70 is in fact a Series 80 pistol with Series 80 trigger system parts, only without the actual firing pin block. This removes from the trigger-feel equation the internal movement that disengages the firing pin block.

Well, there is one other difference. The new Mk. IV Series 70 is produced in very limited runs in the Colt Custom Shop. That means if you find one, you’re laying your hands fairly rare pistol. My source puts the annual production numbers of Colt Custom Shop Mk. IV Series 70s at between 500 and 1,000. Additionally, as these pistols originate from the Custom Shop, they ship in the famous blue “Colt Custom Shop” box, which alone is worth probably north of a hundred bucks.

Standard Colt plastic case next to Colt Custom Shop box

Standard Colt plastic case next to Colt Custom Shop box

Colt Mk. IV Series 70

Colt Mk. IV Series 70

Colt M1991A1

Colt M1991A1

Colt Custom Shop close-up

Colt Custom Shop close-up

Now let’s compare a Mk. IV Series 70 to an M1991A1 Series 80. Externally they appear much the same. Both are roll marked as “Government Models”. Both have nicely checkered rosewood grips. Finish is equally nice on these two examples, and slide-to-frame fit is exceedingly tight, as is the barrel-to-bushing fit. These are very tight pistols which I personally would not hesitate to put up against pistols costing two and three times as much.

Now for the differences:

  • The most obvious are the sights. Both pistols come equipped with High-Profile sights, but the M1991A1 sights use the three-dot system while the Mk. IV Series 70 is absent any such visual cue.
Combat sights vs. three-dot sights

Combat sights vs. three-dot sights

  • Less obvious is the trigger placement. The aluminum M1991A1 trigger is a longish affair protruding almost half way across the span inside the trigger guard. The stainless Mk. IV Series 70 is much shorter in length.
Short Mk. IV Series 70 trigger (top) next to the M1991A1 long trigger

Short Mk. IV Series 70 trigger (top) next to the M1991A1 long trigger

  • Least obvious of all is the backstrap below the grip safety. Both backstraps are grooved in this area, but the M1991A1 backstrap is straight while the Mk. IV Series 70 backstrap has a slight curve.
Mk. IV Series 70 curved backstrap (bottom); M1991A1 flat backstrap (top)

Mk. IV Series 70 curved backstrap (bottom); M1991A1 flat backstrap (top)

On Wednesday we’ll test conventional wisdom and put the Series 70 trigger to the test against the Series 80.

I would like to take this time to acknowledge the fine photo work of Hooper Gun Works, to which I linked in this blog.  Hooper probably explain the Series 70 and Series 80 difference better than I, so please give their wonderful article a look as well by clicking on the link below:

Hooper Gun Works article on the Colt Series 70

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3 Comments

Filed under Firearms, R. Doug Wicker

3 responses to “A Look at the Colt Mk. IV Series 70

  1. Pingback: Anonymous

  2. John

    Have you ever seen a broken collet bushing on one of the 70 series Colts?

    • I personally have not, John. My understanding is that this is a sporadic problem associated with too tight a clearance in some weapons. Reportedly, the collet fingers can (and have) break when the collet fingers lack sufficient room to open. This can happen when there is a collet that’s too wide on the outside diameter, the barrel is slightly oversized, or the slide opening is too small.

      If any of these three things happen, the fingers on the collet flex instead of open. The strain of flexing causes the fingers to break off, and when it does that jams the slide potentially rendering the gun temporarily unusable.