As much as I appreciate hand detachable locks for the easy & no tools required access to the locks & gun interior for drying out & occasional lubrication/cleaning, the breakdowns encountered in the field by me & by people I was with at the time of breakage in the past 40 years of using double guns only amount 6 instances that I can remember.
1.Merkel sidelock with broken mainspring
2.Piotti sidelock with broken top lever spring
3.Piotti sidelock with broken swivel in the left lock.
4.Grulla sidelock with broken top lever spring
5.Purdey sidelock with broken hammer
6.AYA sidelock with broken hammer
Only the Merkel had hand detachable locks & none of the guns had spare locks but the point is that a spare set of locks would have easily & quickly repaired the breakdown in 4 out of the 6 cases.
To me, the value of hand detachable locks depends on under what conditions you are using the gun & how important it is to you to keep your gun working. The ease of repair part of the argument is only valid if you have a spare set of locks for the gun & have them with you (Perazzi mainspring replacement excepted).
Well, for the same outlay of money, one could likely have a spare boxlock as backup to the lock on a sidelock.
Maybe less money if you were willing to vet a boxlock off the continent.
Best,
Ted
