I have no experience with under-lever hammer guns. Are there different things to look for when examining one as opposed to double with hammers inside and top lever opening system. Thanks.

Lefaucheux lever extends from the action and lies when action closed along the top of the forend. An example would be found on the lower end Huskies. The Jones underlever extends from the action and is curved around the trigger guard. Both Lefauchex and Jones open the action by moving the lever laterally. Jones is found on guns from the UK. Greener made use of the Jones lever. I find the Jones far more appealing than the Lefaucheux. I'm sure someone more knowledgeable can correct and add to what I've written. Gil

Thanks. To clarify it is Jones underlever type.

Jones under lever is slower to operate than a top or side lever gun, but is very strong with two bearing surfaces and a positive direct lock up you control in plain sight. Obviously, if hammers are outside you have to cock them to fire, if internal they stay cocked and rely upon the safety. Hammers can be rebounding or not. Non rebounding hammers usually have a half cock which serves as sort of a safety and eases final cocking by taking up part of the spring tension. Rebounding automatically stand clear of the firing mechanism and require a full cocking movement to get into battery. Either hammer mechanism can result in a premature fire when your hot sweaty thumb slips during cocking...Geo

Geo,
A correctly set up rebounding lock SHOULD NOT result in premature fire if the hammer slips off your finger as the sear should slip into the rebounding bent and prevent it unless you are pulling the trigger at the same time. In which case its your own fault!!!
I would stress SHOULD NOT.

That's what I've always thought Mr. Barclay. Thanks for pointing out just how the system is supposed to work. I have seen unintentional discharges though with rebounding hammers.

Could be a finger prematurely on the trigger as you surmised or a defect in the mechanism. While the sear should slip into the rebound bent as you say, wear and the force of the falling hammer could cause the mechanism to fail.

The fails I've seen have all been as a bird flushes or a target is thrown and the shooter attempts to cock the hammer and fire in a hurry. Never seen any damage caused except to the nerves of the shooter and those around...Geo

A slipped hammer should also be caught by the safety notch on a non-rebounding hammer as long as the trigger isn't pulled. Generally speaking though these are more fragile & prone to breakage than are the rebounders.
I really prefer hammerless but have used many hammer guns over the years with no problems. With my small hands though I never attempt to cock Two at a Time as for a double.