Virtually all sidelock guns can be divided into two groups: Back actions, or Back locks, and Bar actions, or Bar locks. The difference lies in the orientation of the main spring of the lock.

Back actions have their spring behind the tumbler and in Bar actions the spring lies along the inside of the action bar, hence their names. The easiest way to identify your type of action is to look at the shape of the lock plate. If it stops in line or behind the line of the breech face, it is a Back action. If however it continues along the length of the action bar, it is most likely a Bar action. However, a final check should be made for the locating pin at the front of the Bar action mainspring which should be clearly visible at the front of the tongue of the lock plate.

J Blanch Back action hammergun no. 5012	J Blanch Bar action hammergun no. 5517
J Blanch Back action hammerless gun no. 6047	J Blanch Bar action hammerless gun no. 6547

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If this pin is not visible there are two possibilities: either you are in possession of a gun with 'pinless' lockplates and this can be confirmed by looking for the other pins on the main body of the lock plate. If they are present then you most likely have a Back action gun masquerading as a Bar action gun. Blanch produced quite a number of these, often with Baker patent self opening actions, and they are usually engraved with good quality bouquet & scroll.

If the pins are not visible anywhere on the lock plate then you may have a very rare gun and should contact the Database immediately!