A number of you have made mention of the pocket or under bore size chamber at the bottom of the barrel asking why it is there. Several Manufactures use this design including Thompson Center.
What you are seeing is called a Patent Breech. It was developed in England as an improvement over the typical flintlock touch hole in the side of the barrel and proved to be worthy of being carried forward into the precussion era.
It is usually associated with a hook tang breech plug but the presence or absence of the hook tang does not really have anything to do with it.
The principle it uses is based on the higher pressures developed in the smaller chamber increases the speed and completeness of the combustion. It also creates a jet of flame into the remaining powder in the barrel rather than just relying on pressure and the flame fronts movement to ignite the rest of the powder charge.
While reading a old issue of MuzzleBlasts (Jan 1997 pp 45-47) I came across a article which may be of some interest to you.
The author built two almost identical .45 cal flintlock rifles, the only difference being the style of the breech.
Both guns had 36 inch Green Mountain Barrels.
He then measured the velocity of each gun with different loads. The following gives the results.
To save typing I will refer to the patent breech with the letter "P" and the standard side ignition with the letter "S".
STYLE__LOAD________VELOCITY__DIFFERENCE
P_____40 FFFg_______1490
S_____40 FFFg_______1450_______-40
P_____50 FFFg_______1670
S_____50 FFFg_______1590_______-80
P_____50 FFg________1580
S_____50 FFg________1460_______-120
P_____60 FFFg_______1855
S_____60 FFFg_______1725_______-130
P_____60 FFg________1675
S_____60 FFg________1560_______-115
P_____70 FFg________1780
S_____70 FFg________1665_______-115
The author also made note that the Patent Breech gun left less fouling in the bore.
If your curious you can use the numbers to see what the change in velocity was for him by changing loads, or changing powder granulation sizes.
What you are seeing is called a Patent Breech. It was developed in England as an improvement over the typical flintlock touch hole in the side of the barrel and proved to be worthy of being carried forward into the precussion era.
It is usually associated with a hook tang breech plug but the presence or absence of the hook tang does not really have anything to do with it.
The principle it uses is based on the higher pressures developed in the smaller chamber increases the speed and completeness of the combustion. It also creates a jet of flame into the remaining powder in the barrel rather than just relying on pressure and the flame fronts movement to ignite the rest of the powder charge.
While reading a old issue of MuzzleBlasts (Jan 1997 pp 45-47) I came across a article which may be of some interest to you.
The author built two almost identical .45 cal flintlock rifles, the only difference being the style of the breech.
Both guns had 36 inch Green Mountain Barrels.
He then measured the velocity of each gun with different loads. The following gives the results.
To save typing I will refer to the patent breech with the letter "P" and the standard side ignition with the letter "S".
STYLE__LOAD________VELOCITY__DIFFERENCE
P_____40 FFFg_______1490
S_____40 FFFg_______1450_______-40
P_____50 FFFg_______1670
S_____50 FFFg_______1590_______-80
P_____50 FFg________1580
S_____50 FFg________1460_______-120
P_____60 FFFg_______1855
S_____60 FFFg_______1725_______-130
P_____60 FFg________1675
S_____60 FFg________1560_______-115
P_____70 FFg________1780
S_____70 FFg________1665_______-115
The author also made note that the Patent Breech gun left less fouling in the bore.
If your curious you can use the numbers to see what the change in velocity was for him by changing loads, or changing powder granulation sizes.
