Bulldog563:

The answer to your question is not as simple as you might think. The short answer is that best quality sidelocks are more expensive than boxlocks of comparable quality mostly due to tradition and, because the market has long been accustomed to this tradition, the market will still bear it.

In true handmade best quality guns, sidelocks do take more bench time to make than boxlocks, but not as much as the price difference often suggests. This is why most of the old-line British makers, who once offered boxlocks as standard fare to fill a market demand, no longer offer them.
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