Describing Chokes

[TARNATION !!!]

I am familiar with the words used to describe choke
Full Modified Cylinder and the improved versions

I am not at all familiar with numerically described chokes
e.g.
chokes - R)618-004, L)620-018

I have also seen 3/4 choke

Can someone fill me in on the numbers please?

Thanks
Jack

[Trigg Davis]

The first number is the size of the bore and the second the amount of constriciton. In a 12 gauge the nomingal bore is 729 but a given gun might be somehat more or somewhat less. In a 12 gauge a constiction of 10 would be IC and 20 modified as an example. A given gun might be 730 as opposed to the theoretical 729 and a constriction of 18 would mean the acrual choke was 712 with a constriction of 18 in the 730 gun whereas a 729 gun with a constrictuion of 18 would be 711.Your numbers appear likely to have come from a 20 gauge.

[Double Play]

Jack,
When you see choke listed in thousandths of an inch as you described (.004&.018) that is the amount of actual constriction between the bore diameter and the muzzle diameter. The reason those cylindrical choke gages are not always accurate is that they assume the gun has the standard bore. Many of these old doubles have been honed out to remove pitting or in some cases to increase restriction and in effect increase choke.

[Austin W Hogan]

Parker guns were "choked to deliver" if the customer requested a particular choke; the customer requests were usually noted as "as close as possible" or "open". The "default pattern" for guns made "on spec" for large orders was F & F and then M & F in the 20th century.
Prior to s/n 70000 the most frequent bore diameter is .750 inch ( plus .010) in 12 ga and the most frequent choke is .032. Note that a twelve ga shell is .810 OD with .030 walls on the paper tube; .810 - .060 = .750, perfect alignment of shell and bore.
A change in bore diameter at around 70000 is quoted in TPS. Dave Suponski measured the internal dimensions of an unfinished barrel and determined that after s/n 70000 a single shaped reamer was used to bore twelve ga barrels. It bored a straight section of about .730 that tapered to .695 at the muzzle. The bores were finished to .730 - .735, and the section near the muzzle was reamed to deliver the desired choke. Note that .735 - .695 is .040, the usual constriction of a full choke. Mark Conrad found that at the time fluid steel began to dominate Parker production at s/n 90000 the Patterning proceedure, and perhaps other things changed in the barrel shop.
The attached sketch shows the interior profiles of some Parker "competition" guns that I measured in preparation for measuring the "Super Fox" at Julia's for the DGJ article. Note that the .005 in/in taper of the chamber reduces the paper case mouth to .740 inside diameter, and that the .740 bore is again a match. The "Super Fox" is chambered and bored similarly.
The Parker forcing cones are 2 - 3 inches long, not quite as long as the Super Fox.
The plotted diameter of the muzzle section of the barrel shows the "OGEE" choke pattern described by Oscar Gaddy, and shown in a drawing in TPS. This was apparently ground into the reamer. Note that the final chokes plotted smooth out the OGEE pattern. The gun with the parabolic profile was marked "retarget before shipping" in the order book; this work was apparently necessary to produce the desired constriction.
Did King/Geary's barrel shop cut these chambers bores and chokes? We will never know. However, these cuts were made from the breech, not from the muzzle as done by most outside shops and gunsmiths.

Best, Austin

[TARNATION !!!]

Thanks gents for the information on the decimal numbers. It makes sense now. Any info for a 3/4 choke description? 3/4 of what?
Thanks,
Jack

[Bill Murphy]

Depending on the gauge, for example, a 12 gauge full is .040, so a half choke is .020, three quarters is .030.

[TARNATION !!!]

Thanks Bill. All the numbers now make sense and to think all I ever used to do was pull the trigger and sometimes pick up a dead bird.
Cheers,
Jack