According to a PTI report, the Naxals who attacked the Congress convoy in Chhattisgarh used archaic weapons from the 1830s and 1840s named Bharmar guns.
A ‘bharmar’ is a muzzle loading firearm, used in the 1830s and 1840s, into which the gunpowder and the explosive are loaded crudely from the barrel of the gun and then fired with an igniting jolt.
The archaic weapon, only reported to be used by Maoists now-a-days in the country, weighed heavy in the gunbattle where personal security officials of Congress leaders were reported to have fired from their modern rifles.
According to police officials, a muzzle-loading gun needs manual replenishment of explosives after every fire and is different from the staccato and precision firing of an AK series rifle or the modern MP5 gun.
A 2009 story on Rediff appears to suggest that while the technology might be archaic, the guns themselves are locally manufactured by the Naxals. A TOI report from earlier this year is rather dismissive of their effectiveness:
The police have seized 212 Bharmar guns or country-made rifles in the last five years. The Bharmar gunss do not have traditional bullets but the ammunition is prepared using traditional gun powder which may not be strong enough to kill humans. In Gondia district - another naval-affected district - the arms haul has been quite low in the last few years. A .303 rifle was seized by the Gondia police in 2008, a sten gun in 2011 and another country-made revolver last year.
Robert Elgood's Firearms of the Islamic World provides some historical background:
An Indian version of the Monteccuccoli lock combining flint and match was invented at the time of Ranjit Singh by a Hindustani named Mirza Bharmar after whom the gun is named in India; an example of the weapon is described by Baden Powell as a Bharmar Lahori, presumably from the place of manufacture. (See Catalogue no. 122.) The gun was popular and Gurkhas were seen armed with this type in 1827.